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nt Times

149

XIII.

How Pictures can be sent by Wire

176

XIV. A Wonderful Example of Science and Skill 191

XV. Scientific Testing and Measuring 198

XVI. Colour Photography212

XVII. How Science aids the Stricken Collier 220

XVIII. How Science helps to keep us well 231

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XIX. Modern Artillery 236

Appendix 245

Index 247

[7]

LIST OF ILLUSTRATIONS

A Huge Lamp Frontispiece facing page

First Effect of the Dynamite 16

A Fine Crop 24

Apple-tree planted by Spade 48

Machine-made Ice 72

A Cold Store 80

Dassen Island Lighthouse 88

Measuring Heat 128

The Telewriter 184

A Miners' Rescue Team 208

Pneumatic Hammer Drill 216

An Artificial Coal Mine 224

Sectional view of a 60-pounder Gun 232

Rifles of different Nations 240

DIAGRAMS

fig. page

1. Principle of Galvanometer 30

2. String Galvanometer 31

3. Duddell Thermo-Galvanometer 39

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4. Construction of a Voltmeter 64

5. The Working of a Refrigerating Machine 70

6. Hertz's Machine 155

7. Hertz "Detector" 156

8. 9. 10. Wireless Waves 158

11. A Wireless Antenna 164

12. Poulsen's Machine 166

13. 14. How Pictures are sent by Wire 177

15. Message received by Telewriter189

[9]

MARVELS OF SCIENTIFIC INVENTION

CHAPTER I

DIGGING WITH DYNAMITE

Most people are afraid of the word explosion and shudder with apprehension at the mention of dynamite. The latter, particularly, conjures up visions of anarchists, bombs, and all manner of wickedness. Yet the time seems to be coming when every farmer will regard explosives, of the general type known to the public as dynamite, as among his most trusty implements. It is so already in some places. In the United States explosives have been used for years, owing to the exertions of the Du Pont Powder Company, while Messrs Curtiss' and Harvey, and Messrs Nobels, the great explosive manufacturers, are busy introducing them in Great Britain.

It will perhaps be interesting first of all to see what this terror-striking compound is. One essential feature is the harmless gas which constitutes the bulk of our atmosphere, nitrogen. Ordinarily one of the most lazy, inactive, inert of substances, this gas will, under certain circumstances, enter into combination with others, and when it does so it becomes in some cases the very reverse of its usual peaceful, lethargic self. It is as if it entered reluctantly into these compounds and so introduced an element of instability into them. It is like a dissatisfied partner in a business, ready to break up the whole combination on very slight provocation.

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And it must be remembered that an explosive is simply some chemical compound which can change suddenly into[10] something else of much larger volume. Water, when boiled, increases to about 1600 times its own volume of steam, and if it were possible

to bring about the change suddenly water would be a fairly powerful explosive. Coal burnt in a fire changes, with oxygen from the atmosphere, into carbonic acid gas, and the volume of that latter which is so produced is much more than that of the combined volumes of the oxygen and coal. When the burning takes place in a grate or furnace we see nothing at all like an explosion, for the simple reason that the change takes place gradually. That is necessarily so since the coal and oxygen are only in contact at the surface of the former. If, however, we grind the coal to a very fine powder and mix it well with air, then each fine particle is in contact with oxygen and can burn instantly. Hence coal-dust in air is an explosive. It used to be thought that colliery accidents were due entirely to the explosion of methane, a gas which is given off by the coal, but it has of recent years dawned upon people that it is the coal-dust in the mine which really does the damage. The explosion of methane stirs up the dust, which then explodes. The former is comparatively harmless, but it acts as the trigger or detonator which lets loose the force pent up in the innocent-looking coal-dust. Hence the greatest efforts in modern collieries are bent towards ridding the workings of dust or else damping it or in some other way preventing it from being stirred up into the dangerous state.

So the essential feature of any explosive is oxygen and something which will burn with it. If it be a solid or liquid the oxygen must be a part of the combination or mixture, for it cannot get air from the surrounding atmosphere quickly enough to explode; and, moreover, it is generally necessary that explosives should work in a confined space away from all contact with air. So oxygen, of necessity, must be an integral part of the stuff itself. But when oxygen combines with anything it usually clings rather tenaciously to its place in the compound and is not easily disturbed quickly, and that is where the nitrogen seems to find its part. It supplies[11] the disturbing element in what would otherwise be a harmonious combination, so that the oxygen and the burnable substances readily split up and form a new combination, with the nitrogen left out.

Of all the harmless things in the world one would think that that sweet, sticky fluid, glycerine, which most of us have used at one time or another to lubricate a sore throat, was the most harmless. As it stands in its bottle upon the domestic medicine shelf, who would suspect that it is the basis of such a thing as dynamite?

Such is the case, however, for glycerine on being brought into contact with a mixture of sulphuric and nitric acids gives birth to nitro-glycerine, an explosive of such sensitivity, of such a furious, violent nature, that it is never allowed to remain long in its primitive condition, but is as quickly as possible changed into something less excitable.

Glycerine is one of those organic compounds which is obtained from once-living matter. Arising as a by-product in the manufacture of soap, it consists, as do so many of the organic substances, of carbon and hydrogen, the atoms of which are peculiarly arranged

to form the glycerine molecule. To this the nitric acid adds oxygen and nitrogen, the sulphuric acid simply standing by, as it were,

and removing the surplus water which arises during the process. So while glycerine is carbon and hydrogen, nitro-glycerine is carbon, hydrogen, nitrogen and oxygen. In this state they form a compact liquid, which occupies little space.

The least thing upsets them, however. The carbon combines with oxygen into carbon dioxide, commonly called carbonic acid gas, the hydrogen and some more oxygen form steam, while the nitrogen is left out in the cold, so to speak. And the total volume of the gases so produced is about 6000 times that of the original liquid. It is easy to see that a substance which is liable suddenly to increase its volume by 6000 times is an explosive of no mean order.

But the fact that it is liable to make this change on a comparatively slight increase in temperature or after a[12] concussion makes it too dangerous for practical use. It needs to be tamed down somewhat. This was first done by the famous Nobel, who mixed it with a fine earth known as kieselguhr, whereby its sensitiveness was much decreased. This mixture is dynamite.

It will be seen that the function of the "earth" is simply to act as an absorbent of the liquid nitro-glycerine, and several other things can be used for the same purpose. Moreover, there are now many explosives of the dynamite nature but differing from it in having an active instead of a passive absorbent, so that the decrease in sensitivity is accompanied by an increase in strength. For example, gelignite, which is being used for agricultural purposes in Great Britain, consists of nitro-glycerine mixed with nitro-cotton, wood-meal and saltpetre. The wood-meal acts as the absorbent instead of the kieselguhr, while the nitro-cotton is another kind of explosive and the saltpetre, one of the ingredients in the old gunpowder, provides the necessary oxygen for burning up the wood-meal. Nitro-cotton is made in much the same way as nitro-glycerine, except that cotton takes the place of the glycerine. Cotton is almost pure cellulose, another organic substance, like glycerine insomuch as it is composed of carbon and hydrogen, but, unlike it, contain-

ing also oxygen. Treated with nitric acid it also forms a combination of carbon, hydrogen, oxygen and nitrogen, which is called nitro-cotton, nitro-cellulose, or gun-cotton.

It may be asked, why, if these two substances are thus similar, need they be mixed? The answer is that although alike to a certain

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degree they are not exactly the same, and the modern manufacturer of explosives in his strife after perfection finds that for certain

purposes one is the best, and for others another, while for others again a combination may excel any single one.

For some work another kind of explosive altogether is to be preferred. This is based upon chlorate of potash, a compound very rich in oxygen, which it is prepared to give[13] up readily to burn any other suitable element which may be at hand. A well-known explosive of this class is that known as cheddite, since it was first made at a factory at Chedde, in Savoy.

For the sake of simplicity, however, I propose in the following descriptions to refer to all these explosives under the common term "dynamite," since that will probably convey to the general public an idea of their nature better than any other term or terms which I could choose.

So now we come to the great question, how can the modern farmer benefit by the use of high explosives such as these? The answer is, in many ways. Let us take the most obvious one first.

A farmer has been ploughing his land and growing his crops upon it for years. Perchance his forefathers have been doing the same for generations. Every year, for centuries possibly, a hard steel ploughshare has gone over that ground, turning over and over the

top soil to a depth of six to eight inches. Each season the plants, whatever they may be, grow mainly in that top layer. They take the goodness or nourishment out of it and it eventually becomes more or less sterile. By properly rotating his crops he mitigates this to

a certain extent, in addition to which he restores to the land some of its old nitrogenous constituents by the addition of manure. Yet, do what he will, this thin top layer is bound to become exhausted. And all the while a few inches lower down there is almost virgin soil which has scarcely been disturbed since the creation of the world.

Nay, more, that virgin soil, with all its plant food still in it, is not only doing little for its owner, it is positively doing him harm. For every time his plough goes over it it tends to ram it down flat; every time a man walks over it the result is the same; every horse that passes, everything that happens or has happened for centuries in that field, tends to make that soil just below the reach of

the ploughshare a hard, impervious mass, through which only the roots of the most strongly growing plants can find[14] a way, and which tends to make the soil above it wet in wet weather and dry in dry weather. Thus roots have to spread sideways instead of downwards; or, growing downwards with difficulty, each plant has to expend vital energy in forcing its roots through the hard

ground which it might better employ in producing flowers or fruits. And there is no natural storage of water. A shower drenches the

ground. In time it dries, through evaporation into the air, and then when the drought comes all is arid as the Sahara.

That hard subsoil is known by the term "hard-pan," and, as we have seen, it is produced more or less by all that goes on in the field. Even worse is the case--a very frequent one too--wherein there is a natural stratum of clay or equally dense waterproof material lying a few feet down.

Beyond the reach of any plough, this hard stratum can be broken up by the use of dynamite. The usual method is to drive holes in the ground about fifteen to twenty feet apart and about three or four feet deep, right into the heart of the hard layer. At the bottom of each hole is placed a cartridge of dynamite with a fuse and a detonator. This latter is a small tube containing a small quantity of explosive which, unlike the dynamite, can be easily fired, and initiates the detonation of the cartridge.

When these miniature earthquakes have taken place all over a field a very different state of things prevails. The "hard-pan" has been broken. The explosive used for such a purpose has a sudden shattering power, whereby it pulverises the ground in its vicinity rather than making a great upheaval at the surface. The sudden shock makes cracks and fissures in all directions, through which roots can easily make their way. Moreover, it permits air to find an entrance, thereby aerating the soil in such a way as to increase its fertility. The heat, or else the chemical products of the explosion, seem to destroy the fungus germs in the ground. Finally a natural storage of water is set up. Heavy rain, instead of drenching the upper soil, simply moistens it nicely, while the surplus water descends into the newly disturbed[15] layers, there to remain until the roots pump it up in time of drought.

It is stated that an acre of hay pumps up out of the soil 500 tons of water per annum, so it is easy to see what an important feature this natural water-storage is.

Farmers say that their crops have doubled in value after thus dynamiting the subsoil.

This operation has been spoken of as a substitute for ploughing, but that may be put down to "journalistic licence," for while it truly conveys the general idea, it is hardly correct. The ordinary plough turns over about eight inches, the special subsoil plough reaches down to about eighteen inches, but the dynamite method loosens the ground to a depth of six or seven feet. Corn roots if given a chance will go downwards from four to eight feet. Potatoes go down three feet, hops eight to eighteen feet and vines twenty feet, so it is easy to see how restricted the plants are when their natural rooting instincts are restrained by a hard layer at a depth of eighteen

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inches or so.

The holes are made by means of a bar or drill. A great deal depends, of course, upon the hardness of the soil. Sometimes a steel bar has to be driven in by a sledge-hammer. At others a pointed bar can be pushed down by hand. In some cases it will be found that the best tool to employ is a "dirt-auger," a tool like a carpenter's auger, which on being turned round and round bores its way into the earth. However it may be done, one or more cartridges of dynamite are lowered into the finished hole, one of them being fitted with the necessary detonator and fuse. Then a little loose earth or sand is dropped into the hole until it is filled to a depth of six inches or so above the uppermost cartridge. Above that it is quite safe to fill the hole with earth, ramming it in with a wooden rammer. This is called "tamping," and it is necessary in order to prevent the force of the explosion being wasted in simply blowing up the hole. What is wanted is that the explosion shall take place within an enclosed chamber so that its effect may be felt equally[16] in all directions. The holes are generally about an inch and a half or an inch and three-quarters in diameter.

There are two ways of firing the charges. One is by means of fuses. The detonator is fastened to one cartridge and a length of fuse is attached to the detonator, which passing up the hole terminates above the ground. The fuse is a tube of cotton filled with gunpowder, and it burns at the rate of about two feet a minute. Thus if three feet of fuse be used the man who lights it has a minute and a half in which to find a place of safety from falling stones.

The other way is by electricity. In this case an electric fuse is attached to the cartridge and two wires are led up the hole. These are connected to an electrical machine, which causes a current to pass down into the fuse, where, by heating a fine platinum wire, it fires the detonating material with which it is packed. This detonating material in turn fires the dynamite.

The advantage of the electrical method is that twenty or thirty holes being simultaneously connected to the same machine can all be

fired at once.

And now let us think of another kind of farming, in which fruit trees are concerned. With a large tree the need of plenty of underground space for its roots would seem to be more important even than in the case of annual plants like wheat. Yet we know very

well that the usual procedure is to dig a small hole just about big enough to accommodate the roots of the sapling when it is planted, while the ground all round is left undisturbed. The assumption is that the tree will, in time, be able to push its roots through anything which is not actually solid rock. So much is this the case that one authority has thought fit to warn tree-growers in this picturesque fashion. "When planting a tree," he says, "forget what it is you are doing, and think that you are about to bury the biggest horse you know." How many people when planting any tree dig a hole big enough to bury a horse? It is fairly safe to reply, only those who do it by dynamite.

By permission of Dupont Powder Co., Wilmington, Delaware

First Effect of the Dynamite

Clearing a field of tree stumps by blowing them up with dynamite.--See p. 16

[17]

The method of working is to bore a hole nearly as deep as the hole you want to blast. At the bottom place a powerful charge, far stronger than you would use for "subsoiling," as just described. That will not only blow a hole big enough for you to put your tree in, but it will loosen the ground all around the hole for yards. The main debris from the hole will fall back into it, but that will not

matter much, since, being all loose, it is an easy matter to remove as much as is necessary to plant the young tree. The advantages are the same as those enumerated in the previous case--namely, the loosened ground gives more scope for the roots--apple-tree roots want twenty feet or so--the ground holds moisture better, and the explosion kills the fungus germs. In addition to these there is the advantage that to blast a hole like this is cheaper than digging it.

And that the advantages are not merely theoretical is shown by the fact that trees so planted actually do grow stronger, bigger and quicker than precisely similar ones under the same conditions, but set in the ordinary way with a spade.

And not only do new trees thus benefit; old trees can be helped by dynamite. Many an existing orchard has been improved by exploding dynamite at intervals between the rows of trees. Care has to be taken to see that the disturbance is not so violent or so close as to damage the trees, but that can be easily arranged, and then the result is that the soil all around the trees is loosened, the roots are given more freedom and the water-storing properties of the ground are greatly improved.

Again, how often a farmer is troubled with a pond or a patch of marshy ground right in the midst of his fields. It is of no use,

and simply serves to make the field in which it occurs more difficult to plough and to cultivate--besides being so much good land wasted. Now the reason for the existence of that pond or marsh is that underneath the surface there is an impervious layer in which, as in a basin, the water can collect. Make a hole in that and it will no[18] more hold water than a cracked jug will. And to make that

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hole with dynamite is the easiest thing in the world.

If the pond be merely a collection of water which occurs in wet weather, but which dries up quickly, there simply needs to be drilled a deep hole and a fairly strong explosion caused at the bottom of it. How deep the hole must be depends upon the formation of the earth at that point, and how low down is the stratum which, being waterproof, causes the water to remain. It is that, of course, which must be broken through, and so the explosion must be caused at a point near the under side of that layer. With a little experience

the operator can judge the position by the feel of the tool with which he makes the hole. If the pond is permanent but shallow, men can wade to about the centre, there to drill a hole and fire a shot. If it be permanent and deep, then the work must be done from a raft, which, however, can be easily constructed for the purpose. Once broken through, the water will quickly pass away below the impervious stratum and useless land will become valuable.

The same may be done on a larger scale by blasting ditches with dynamite. This is in many cases much cheaper than digging them. A row of holes is put down, or even two or three rows, according to the width of the proposed ditch. In depth they are made a lit-tle less than the depth of the ditch that is to be. And for a reason which will be apparent they are put very close together, say three feet or so apart. Preparations may thus be made for blasting a ditch hundreds of feet long and then all are fired together. The earth is thrown up by a mighty upheaval, a ditch being produced of remarkable regularity considering the means by which it is made.

The sides, of course, take a nice slope, the debris is thrown away on both sides and spread to a considerable distance, so that, given favourable conditions and a well-arranged explosion, there is constructed a finished ditch which hardly needs touching with spade or other tool.

It not being feasible to fire a lot of holes electrically, the limit being about thirty, the simultaneous explosion of[19] perhaps hundreds has to be brought about in some other manner, and usually it is accomplished by the simple device of putting the holes fairly near together and firing one with a fuse. The commotion set up by this one causes the nearest ones to "go off," they in turn detonating those farther on, with the result that explosion follows explosion all along the line so rapidly as to be almost instantaneous.

A farmer who is troubled by a winding stream passing through his land, cutting it up into awkward shapes, can straighten it by blasting a ditch across a loop in the manner just described. In the case of lowlying land, however, ditches are obviously no use, since water would not flow away along them. In that case the principle suggested just now for dealing with an inconvenient pond can sometimes be used, for if the subsoil be blasted through at several points it is very likely that water will find a way downwards by some means or other.

And the list of possible uses is by no means exhausted yet. A man opening up virgin land often finds old tree stumps his greatest bother. He can dig round them and then pull them out with a team of horses, but by far the simpler way is with a few well-placed dynamite cartridges, for they not only throw up the stump for him, but they break it up, shake the earth from it, and leave it ready for him to cart away or to burn.

Boulders, too, can be blown to pieces far more easily than one would think. The charges may be put underneath them as with the tree stumps, but in many cases that is not necessary, all that is needed being some dynamite laid upon the top of the rock and covered with a heap of clay. So sudden is the action of the explosive that its shock will break up the stone underneath it. Yet another way, perhaps the most effective of all, is to drill a hole into the stone and fire a charge inside it. It behoves the onlooker then to keep away, for the fragments may be thrown three or four hundred feet, a fair proof that the stone will be very thoroughly demolished.

[20]Even in the digging of wells explosives may be useful. In that case the holes are made in a circle, and they slant downwards and inwards, so that their lower ends tend to meet. The result of simultaneously exploding the charges in these holes is to cut out a conical hole a little larger in size than the ring and a little deeper than the point at which the explosion took place. The bottom of that hole can be levelled a little and the operation repeated, and so stage by stage the well will proceed to grow downwards.

The thought that naturally occurs to one is this. All the operations described may be very well, the cost may be low, and the effect good, but are they sufficient to compensate for the risks necessarily dependent upon the use of explosives? The doubt implied in that question, natural though it be, is based upon prejudice, with which we are all more or less afflicted. The art of making these explosive substances has been brought to such a pitch that with reasonable care there is no risk whatever. The greatest possible care is used in the factory to see that all explosives sent out are what they are meant to be, and that they can therefore be relied upon to behave according to programme and not to play any tricks. That is the first step, and what with competition between makers, Gov-

ernment inspection, and searching inquiry into the slightest accident, and the desire of each maker to keep up the credit of his name, it is safe to say that modern explosives may be relied upon to do their duty faithfully. The second step in the process of securing safety is that the powerful explosive, the one that does the work, is made very insensitive, so that it is really quite hard to explode it. With reasonable care, then, it will never go off by accident. On the other hand, the sensitive material, which is easy to "let off," is in very small quantities, so small that an accident with it would not, again with reasonable precautions, be a serious matter.

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Fuse, too, is very reliable nowadays. The man who lights the fuse may be absolutely sure that he will have that time to get to a place of safety which corresponds to the length[21] of fuse which he employs. With electrical firing, too, it is quite easy to arrange that the final electrical connection shall not be made until all are at a safe distance, so that a premature explosion is impossible.

In many of the cases described, the shock takes place almost entirely within the earth and there is very little debris thrown about. Indeed the only danger which is to be feared with these operations is about on a par with that which every farm hand runs from the

kick of a horse. Any careful, trustworthy man could be quite safely taught to do this work, and with the assistance of a labourer he

could do all that is necessary. Given a fair amount of intelligence, too, he would take but little teaching. Altogether there is no doubt that the use of explosives is going to have a marked effect upon farming operations in the near future.

[22] CHAPTER II

MEASURING ELECTRICITY

There are many people whose acquaintance with electricity consists mainly in paying the electric light bill. To such the instruments whereby electricity is measured will make a specially interesting appeal.

Current is sold in Great Britain at so much per Board of Trade Unit. To state what that is needs a preliminary explanation of the other units employed in connection with electric currents.

The public electricity supply in any district is announced to be so many volts, it may be 100, 200 or perhaps 230, but whatever it be, it is always so many "volts." Then the electrician speaks lightly of numbers of "amperes," he may even talk of the number of "watts" used by the lamps, while occasionally the word "ohm" will leak out. Among these terms the general reader is apt to become completely fog-bound. But really they are quite simple if once understood, and, as we shall see in a moment, there are some very remarkable instruments for measuring them, some of which exhibit a delicacy truly astonishing.

It is well at the outset to try and divest ourselves of the idea that there is anything mysterious or occult about electricity. It is quite true that there are many things about it very little understood even by the most learned, but for ordinary practical purposes it may be regarded as a fluid, which flows along a wire just as water flows along a pipe. The wire is, electrically speaking, a "hole" through the air or other non-conducting substance with which it is surrounded. A water-pipe being a hole through a bar of[23] iron, so the cop-per core of an electrical wire is, so far as the current is concerned, but a hole through the centre of a tube of silk, cotton, rubber or whatever it be. Electricity can flow through certain solids just as water can flow through empty space.

Water will not flow through a pipe unless a pressure be applied to it somewhere. In a pipe the ends of which are at the same level water will lie inert and motionless. Lower one end, however, and the pressure produced by gravity--in other words, the weight of the water--will cause it to move. In like manner pressure produced by the action of a pump will make water flow. On the other hand, when it moves it encounters resistance, through the water rubbing against the walls of the pipe.

Similarly, an electrical pressure is necessary before a current of electricity will flow. And every conductor offers more or less resistance to the flow of current, thus opposing the action of the pressure. Before current will flow through your domestic glow-lamps and cause them to give light there must be a pressure at work, and that pressure is described as so many volts.

A battery is really a little automatic electrical pump for producing an electrical pressure. And the volt, which is a legal measure, just as much as a pound or a yard, is a certain fraction of the pressure produced by a certain battery known as Clark's Cell. It is not necessary here to say exactly what that fraction is, but it will give a general idea to state that the ordinary Leclanche or dry cell, such as is used for electric bells, produces a pressure of about one and a half volts.

Thus we see the volt is the electrical counterpart of the term "pound per square inch" which is used in the case of water pressure.

A flow of water is measured in gallons per minute. An electrical current is measured in coulombs per second. Thus the coulomb is the electrical counterpart of the gallon. But in this particular we differ slightly in our methods of talking of water and electricity. Gallons per minute or per[24] hour is the invariable term in the former case, but in the latter we do not speak of coulombs per second, although that is what we mean, for we have a special name for one coulomb per second, and that same is ampere. One ampere is one coulomb per second, two amperes are two coulombs per second, and so on.

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By permission of Dupont Powder Co. A Fine Crop

Celery grown on soil tilled by dynamite.--See p. 24

There is no recognised term to denote the resistance which a water-pipe offers to the passage of water through it, but in the similar case with electricity there is a term specially invented for the purpose, the ohm. Legally it is the resistance of a column of mercury of a certain size and weight. A rough idea of it is given by the fact that a copper wire a sixteenth of an inch thick and 400 feet long has a resistance of about one ohm.

The three units--the volt, ampere and ohm--are so related that a pressure of one volt acting upon a circuit with a resistance of one ohm will produce a current of one ampere.

A current can do work; when it lights or heats your room or drives a tramcar it is doing work; and the rate at which a current does work is found by multiplying together the number of volts and the number of amperes. The result is in still another unit, the watt. And 1000 watts is a kilowatt. Finally, to crown the whole story, a kilowatt for one hour is a Board of Trade unit.

So for every unit which you pay for in the quarterly bill you have had a current equal to 1000 watts for an hour. To give a concrete example, if the pressure of your supply is 200 volts, and you take a current of five amperes for an hour, you will have consumed one B.T.U.

Perhaps it will give added clearness to this explanation to tabulate the terms as follow:-- Volt = The unit of pressure, analogous to "pounds per square inch" in the case of water. Coulomb = The measure of quantity, analogous to the gallon.

Ampere = The measure of the "strength" of a current, meaning one coulomb per second. [25]

Watt = The unit denoting the power for work of any current. It is the result of multiplying together volts and amperes. Kilowatt = 1000 watts.

Board of Trade Unit = A current of one kilowatt flowing for one hour.

In practice the measurements are generally made by means of the connection between electricity and magnetism. A current of

electricity is a magnet. Whenever a current is flowing it is surrounded by a region in which magnetism can be felt. This region is

called the magnetic field, and the strength of the field varies with the strength that is the number of amperes in the current. If a wire carrying a current be wound up into a coil it is evident that the magnetic field will be more intense than if the wire be straight, for it will be concentrated into a smaller area. Iron, with its peculiar magnetic properties, if placed in a magnetic field seems to draw the magnetic forces towards itself, and consequently, if the wire be wound round a core of iron, the magnetism due to the current will

be largely concentrated at the ends of the core. But the main principle remains--in any given magnet the magnetic power exhibited

will be in proportion to the current flowing.

The switchboard at a generating station is always supplied with instruments called ammeters, an abbreviation of amperemeters, for the purpose of measuring the current passing out from the dynamos. Each of these consists of a coil of wire through which the current passes. In some there is a piece of iron near by, which is attracted more or less as the current varies, the iron being pulled back by a spring and its movement against the tension of the spring being indicated by a pointer on a dial.

In others the coil itself is free to swing in the neighbourhood of a powerful steel magnet, the interaction between the electro-mag- net, or coil, and the permanent magnet being such that they approach each other or recede from each other[26] as the current varies. A pointer on a dial records the movements as before.

In yet another kind the permanent magnet gives way to a second coil, the current passing through both in succession, the result be-ing very much the same, the two coils attracting each other more or less according to the current.

Another kind of ammeter known as a thermo-ammeter works on quite a different principle. It consists of a piece of fine platinum

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wire which is arranged as a "shunt"--that is to say, a certain small but definite proportion of the current to be measured passes through it. Now, being fine, the current has considerable difficulty in forcing its way through this wire and the energy so expended becomes turned into heat in the wire. It is indeed a mild form of what we see in the filament of an incandescent lamp, where the en-ergy expended in forcing the current through makes the filament white-hot. The same principle is at work when we rub out a pencil mark with india-rubber, whereby the rubber becomes heated, as most of us have observed. The wire, then, is heated by the current passing through it, and accordingly expands, the amount of expansion forming an indication of the current passing. The elongation of the wire is made to turn a pointer.

A simple modification makes any of these instruments into a voltmeter. This instrument is intended to measure the force or pressure

in the current as it leaves the dynamo.

A short branch circuit is constructed, leading from the positive wire near the dynamo to the negative wire, or to the earth, where the pressure is zero. In this circuit is placed the instrument, together with a coil made of a very long length of fine wire so that it has a very great resistance. Very little current will flow through the branch circuit because of the high resistance of the coil, but what there is will be in exact proportion to the pressure. The voltmeter is therefore the same as the ammeter, except that its dial is marked for volts instead of for amperes, and it has to be provided with the resistance coil.

[27]Instruments of the ammeter type can also be used as ohmmeters. In this case what is wanted is to test the resistance of a circuit,

and it is done by applying a battery, the voltage of which is known, and seeing how much current flows.

All the voltmeters and ohmmeters mentioned owe their method of working to what is known as Ohm's law. One of the great-

est steps in the development of electrical science was taken when Dr Ohm put forward the law which he had discovered whereby pressure, current and resistance are related. The reader will probably have noticed from what has already been said about the units of measurement--the volt, the ampere and the ohm--that the current varies directly as the pressure and inversely as the resistance. That is the famous and important "Ohm's law" and anyone who has once grasped that has gone a long way towards understanding many of the principal phenomena of electric currents.

But the instruments so far referred to are of the big, clumsy type, suitable for measuring large currents and great pressures. They are like the great railway weigh-bridges, which weigh a whole truck-load at a time and are good enough if they are true to a quarter of

a hundredweight. The instruments about to be described are more comparable with the delicate balance of the chemist, which can detect the added weight when a pencil mark is made upon a piece of paper. Indeed beside them such a balance is quite crude and clumsy. They may be said to be the most delicate measuring instruments in existence.

We will commence with the galvanometer. The simplest form of this is a needle like that of a mariner's compass very delicately suspended by a thin fibre in the neighbourhood of a coil of wire. The magnetic field produced by the current flowing in the wire tends to turn the needle, which movement is resisted by its natural tendency to point north and south. Thus the current only turns the needle a certain distance, which distance will be in proportion to[28] its strength. The deflection of the needle, therefore, gives us a measure of the strength of the current.

But such an instrument is not delicate enough for the most refined experiments, and the improved form generally used is due to that prince of inventors, the late Lord Kelvin. He originally devised it, it is interesting to note, not for laboratory experiments, but for practical use as a telegraph instrument in connection with the early Atlantic cables.

Before describing it, it may sharpen the reader's interest to mention a wonderful experiment which was made by Varley, the famous electrician, on the first successful Atlantic cable. He formed a minute battery of a brass gun-cap, with a scrap of zinc and a single drop of acidulated water. This he connected up to the cable. Probably there is not one reader of this book but would have thought, if he had been present, that the man was mad. What conceivable good could come of connecting such a feeble source of electrical pressure to the two thousand miles of wire spanning the great ocean; the very idea seems fantastic in the extreme. Yet that tiny battery was able to make its power felt even over that great distance, for the Thomson Mirror Galvanometer was there to detect it. Two thousand miles away, the galvanometer felt and was operated by the force generated in a battery about the size of one of the capital letters on this page.

This wonderful instrument consisted of a magnet made of a small fragment of watch-spring, suspended in a horizontal position by means of a thread of fine silk, close to a coil of fine wire. When current flowed through the coil the magnetic field caused the watch-spring magnet to swing round, but when the current ceased the untwisting of the silk brought it back to its original position again.

So far it seems to differ very little from the ordinary galvanometer previously mentioned, but the stroke of genius was in the method of reading it. With a small current the movement of the magnet was too small to be observed by the unaided eye, so it was attached

9

to a minute mirror[29] made of one of those little circles of glass used for covering microscope slides, silvered on the back. The magnet was cemented to the back of this, yet both were so small that together their weight was supported by a single thread of cocoon silk. Light from a lamp was made to fall upon this mirror, thereby throwing a spot of light upon a distant screen. Thus the slightest movement of the magnet was magnified into a considerable movement of the spot of light. The beam of light from the mirror to the screen became, in fact, a long lever or pointer, without weight and without friction.

The task of watching the rocking to and fro of the spot of light was found to be too nerve-racking for the telegraph operators, and so Lord Kelvin improved upon his galvanometer in two ways. He first of all managed to give it greater turning-power, so that, actu-ated by the same current, the new instrument would work much more strongly than the older one. Then he utilised this added power to move a pen whereby the signals were recorded automatically upon a piece of paper. The new instrument is known as the Siphon Recorder.

The added power was obtained by turning the instrument inside out, as it were, making the coil the moving part and the permanent magnet the fixed part. This enabled him to employ a very powerful permanent magnet in place of the minute one made of watch-spring. The interaction of two magnets is the result of their combined strength, and that of the coil being limited by the strength of the minute current the only way to increase the combined power of the two was to substitute a large powerful magnet for the small magnetised watch-spring. This large magnet would, of course, have been too heavy to swing easily and therefore the positions had to be reversed.

So now we have two types of galvanometer, both due originally to the inventions of Lord Kelvin. For some purposes the Thomson type (his name was Thomson before he became Lord Kelvin) are still used, but in a slightly elaborated form. Its sensitiveness is such that a current of a thousandth of[30] a micro-ampere will move the spot of light appreciably. And when one comes to consider that a micro-ampere is a millionth part of an ampere this is perfectly astounding.

But there is a more wonderful story still to come, of an instrument which can detect a millionth of a micro-ampere, or one millionth of a millionth of an ampere. It is not generally known that we are all possessors of an electric generator in the form of the human heart, but it is so, and Professor Einthoven, of Leyden, wishing to investigate these currents from the heart, found himself in need

of a galvanometer exceeding in sensitiveness anything then known. Even the tiny needles or coils with their minute mirrors have some weight and so possess in an appreciable degree the property of inertia, in virtue of which they are loath to start movement and, having started, are reluctant to stop. This inertia, it is easy to see, militates against the accurate recording of rapid variations in minute currents, so the energetic Professor set about devising a new galvanometer which should answer his purpose. This is known as the "String Galvanometer."

Fig. 1.-This shows the principle of this wonderful Galvanometer invented by Lord Kelvin in its latest form. Current enters at a, passes round the coils, as shown by the arrows, and away at b. A light rod, c, is suspended by the fine fibre, d, so that the eight little magnets hang in the centres of the coils--four in each. The current deflects these magnets and so turns the mirror, m, at the bottom of the rod. At e are two large magnets which give the little ones the necessary tendency to keep at "zero."

The main body of the instrument is a large, powerful electro-magnet, in shape like a large pair of jaws nearly shut. Energised by a strong current, this magnet produces an exceedingly strong magnetic field in the small space between the "teeth" as it were. In this space there is stretched a fine thread of quartz which is almost perfectly elastic. It is a non-conductor, however, so it is covered with a fine[31] coating of silver. Silver wire is sometimes used, but no way has yet been found of drawing any metallic wire so thin as the quartz fibre, which is sometimes as thin as two thousandths of a millimetre, or about a twelve-thousandth of an inch. A hundred pages of this book make up a thickness of about an inch, so that one leaf is about a fiftieth of an inch. Consequently the fibre in question could be multiplied 240 times before it became as stout as the paper on which these words are printed.

Fig. 2.--Here we see the working parts of the "String Galvanometer," by which the beating of the heart can be registered electrically. The current flows down the fine silvered fibre, between the poles, a and b, of a powerful magnet. As the current varies, the fibre bends more or less.

The current to be measured, then, is passed through the stretched fibre and the interaction of the magnetic field by which the fibre is then surrounded, with the magnetic field in which it is immersed, causes it to be deflected to one side. Of course the deflection is exceedingly small in amount, and as it is undesirable to hamper its movements by the weight of a mirror, no matter how small, some other means of reading the instrument had to be devised. This is a microscope which is fixed to one of the jaws, through a fine

hole in which the movements of the fibre can be viewed. Or what is often better still, a picture of the wire can be projected through the microscope on to a screen or on to a moving photographic plate or strip of photographic paper. In the latter case a permanent record is made of the changes in the flowing current.

An electric picture can thus be made of the working of a man's heart. He holds in his hands two metal handles or is in some other

way connected to the two ends of the fibre by wires just as the handles of a shocking coil are connected to the ends of the coil.

10

The faint currents caused by the beating of his heart are thus set down in the form of[32] a wavy line. Such a diagram is called a "cardiogram," and it seems that each of us has a particular form of cardiogram peculiar to himself, so that a man could almost be recognised and distinguished from his fellows by the electrical action of his heart.

The galvanometer has a near relative, the electrometer, the astounding delicacy of which renders it equally interesting. It is particularly valuable in certain important investigations as to the nature and construction of atoms.

The galvanometer, it will be remembered, measures minute currents; the electrometer measures minute pressures, particularly those of small electrically charged bodies.

Every conductor (and all things are conductors, more or less) can be given a charge of electricity. Any insulated wire, for example,

if connected to a battery will become charged--current will flow into it and there remain stationary. And that is what we mean by a

charge as opposed to a current.

Air compressed into a closed vessel is a charge. Air, however compressed, flowing along a pipe would be better described as a current.

Imagine one of those cylinders used for the conveyance of gas under pressure and suppose that we desire to find the pressure of the gas with which it is charged. We connect a pressure-gauge to it, and see what the finger of the gauge has to say. What happens is that gas from the cylinder flows into the little vessel which constitutes the gauge and there records its own pressure.

And just the same applies with electrometers. Precisely as the pressure-gauge measures the pressure of air or gas in some vessel, so the electrometer measures the electrical pressure in a charged body.

Further, some of the charged bodies with which the student of physics is much concerned are far smaller than can be seen with the most powerful microscope. How wonderfully minute and delicate, therefore, must be the instrument which can be influenced by the tiny charge which so small a body can carry.

[33]It will be interesting here to describe an experiment performed with an electrometer by Professor Rutherford, by which he determined how many molecules there are in a centimetre of gas, a number very important to know but very difficult to ascertain, since molecules are too small to be seen. This number, by the way, is known to science as "Avogadro's Constant."

Everyone has heard of radium, and knows that it is in a state which can best be described as a long-drawn-out explosion. It is always shooting off tiny particles. Night and day, year in and year out, it is firing off these exceedingly minute projectiles, of which there are two kinds, one of which appears to be atoms of helium.

Some years ago, when radium was being much talked about and the names of M. and Madame Curie were in everyone's mouth, little toys were sold, the invention, I believe, of Sir William Crookes, called spinthariscopes. Each of these consisted of a short brass tube with a small lens in one end. Looking through the lens in a dark room, one could see little splashes of light on the walls of the tube. Those splashes were caused by a tiny speck of radium in the middle of the tube, the helium atoms from which, by bombarding the inner surface of the tube, produced the sparks.

Now if we can count those splashes we can tell how many atoms of helium are being given off per minute. And if then we reckon how many minutes it takes to accumulate a cubic centimetre of helium we can easily reckon how many atoms go to the cubic centimetre. But the difficulty is to count them.

So the learned Professor called in the aid of the electrometer. He could not count all the atoms shot off, so he put the piece of ra-dium at one end of a tube and an electrometer at the other. Every now and then an atom shot right through the tube and out at the farther end. And since each of these atoms from radium is charged with electricity, each as it emerged operated the electrometer. By simply watching the twitching of the instrument, therefore, [34]it was possible to count how many atoms shot through the tube-- one atom one twitch. And from the size and position of the tube it was possible to reckon what proportion of the whole number shot off would pass that way.

The result of the experiment showed that there are in a cubic centimetre of helium a number of atoms represented by 256 followed by seventeen noughts. And as helium is one of the few substances in which the molecule is formed of but one atom, that is also the number of molecules.

And now consider this, please. A cubic centimetre is about the size of a boy's marble. That contains the vast number of molecules

11

just mentioned. And the electrometer was able to detect the presence of those one at a time. Need one add another word as to the inconceivable delicacy of the instrument.

In its simplest form the electrometer is called the "electroscope." Two strips of gold-leaf are suspended by their ends under a glass

or metal shade. As they hang normally they are in close proximity. Their upper ends are, in fact, in contact and are attached to a small vertical conductor. A charge imparted to the small conductor will pass down into the leaves, and since it will charge them both they will repel each other so that their lower ends will swing apart. Such an instrument is very delicate, but because of the extreme thin-ness of the leaves it is very difficult to read accurately the amount of their movement and so to determine the charge which has been given to them.

In a more recent improvement, therefore, only one strip of gold-leaf is used, the place of the other being taken by a copper strip. The whole of the movement is thus in the single gold-leaf, as the copper strip is comparatively stiff, and it is possible to arrange for the movement of this one piece of gold-leaf to be measured by a microscope.

The other principal kind of electrometer we owe, as we do the galvanometers, to the wonderful ingenuity of Lord Kelvin. In this the moving part is a strip of thin aluminium, which is suspended in a horizontal position by means,[35] generally, of a fine quartz fibre. Since it is necessary that this fibre should be a conductor, which quartz is not, it is electro-plated with silver. Thus a charge communicated to the upper end of the fibre, where it is attached to the case, passes down to the aluminium "needle," as it is called. Now the needle is free to swing to and fro, with a rotating motion, between two metal plates carefully insulated. Each plate is cut into four quadrants, the opposite ones being electrically connected, while all are insulated from their nearest neighbours. One set of quadrants is charged positively, and one set negatively, by a battery, but these charges have no effect upon the needle until it is itself charged.

As soon as that occurs, however, they pull it round, and the amount of its movement indicates the amount of the charge upon the needle, and therefore the pressure existing upon the charged body to which it is connected. The direction of its movement shows, moreover, whether the charge be positive or negative.

A little mirror is attached to the needle, so that its slightest motion is revealed by the movement of a spot of light, as in the case of the mirror galvanometers. Instruments such as these are called "Quadrant Electrometers."

My readers will remember, too, the "String Galvanometer" already mentioned. The same idea has been adapted to this purpose. A fine fibre is stretched between two charged conductors while the fibre is itself connected to the body whose charge is being measured. The charge which it derives from the body causes it to be deflected, which deflection is measured by a microscope.

In all cases of transmission of electricity over long distances for lighting or power purposes the currents are "alternating." They flow first one way and then the other, reversing perhaps twenty times a second, or it may be two hundred, or even more times in that

short period. Some electric railways are worked with alternating current, and it is used for lighting quite as much as direct current and is equally satisfactory.

[36]In wireless telegraphy it is essential. In that case, however, the reversals may take place millions of times per second. Consequently, to distinguish the comparatively slowly changing currents of a "frequency" or "periodicity" of a few hundreds per second from these much more rapid ones, the latter are more often spoken of as electrical oscillations. And these alternating and oscillating currents need to be measured just as the direct currents do. Yet in many cases the same instruments will not answer. There has therefore grown up a class of wonderful measuring instruments specially designed for this purpose, by which not only does the station engineer know what his alternating current dynamos are doing, but the wireless operator can tell what is happening in his apparatus, the investigator can probe the subtleties of the currents which he is working with, and apparatus for all purposes can be designed

and worked with a system and reason which would be impossible but for the possibility of being able to measure the behaviour of the subtle current under all conditions.

One trouble in connection with measuring these alternating currents is that they are very reluctant to pass through a coil.

One method by which this difficulty can be overcome has been mentioned incidentally already. I refer to the heating of a wire

through which current is passing. This is just the same whether the current be alternating or direct.

One of the simplest instruments of this class has been appropriated by the Germans, who have named it the "Reiss Electrical Thermometer," although it was really invented nearly a century ago by Sir William Snow Harris. It consists of a glass bulb on one end of

a glass tube. The current is passed through a fine wire inside this bulb, and as the wire becomes heated it expands the air inside the

bulb. This expansion moves a little globule of mercury which lies in the tube, and which forms the pointer or indicator by which

the instrument is read. As the temperature of the wire rises the mercury is forced away from the[37] bulb, as the temperature falls it returns. And as the temperature is varied by the passage of the current, so the movement of the mercury is a measure of the current.

12

Another way is to employ a "Rectifier." This is a conductor which has the peculiar property of allowing current to pass one way but not the other. It thus eliminates every alternate current and changes the alternating current into a series of intermittent currents all in the same direction. Rectified current is thus hardly described by the term continuous, but still it is "continuous current" in the sense that the flow is always in the same direction, and so it can be measured by the ordinary continuous current instruments. The difficulty about it is that there is some doubt as to the relation between the quantity of rectified current which the galvanometer

registers and the quantity of alternating current, which after all is the quantity which is really to be measured. How the rectification is

accomplished will be referred to again in the chapter on Wireless Telegraphy.

But to return to the thermo-galvanometers, as those are termed which ascertain the strength of a current by the heat which it produces, the simple little contrivance of Sir William Snow Harris has more elaborate successors, of which perhaps the most interesting are those associated with the name of Mr W. Duddell, who has made the subject largely his own. Besides their interest as wonderfully delicate measuring instruments, these have an added interest, since they introduce us to another strange phenomenon in electricity.

We have just noted the fact that electricity causes heat. Now we shall see the exact opposite, in which heat produces electrical pressure and current. And the feature of Mr Duddell's instruments is the way in which these two things are combined. By a roundabout but very effective way he rectifies the current to be measured, for he first converts some of the alternating current into heat and then converts that heat into continuous current.

If two pieces of dissimilar metals be connected together by their ends, so as to form a circuit, and one of the joints be[38] heated, an electrical pressure will be generated which will cause a current to flow round the circuit. The direction in which it will flow will depend upon the metals employed. The amount of the pressure will also depend upon the metals used, combined with the temperature of the junctions. With any given pair of metals, however, the force, and therefore the volume of current, will vary as the temperature. Really it will be the difference in temperature between the hot junction and the cold junction, but if we so arrange things that the cold junction shall always remain about the same, the current which flows will vary as the temperature of the hot one. The volume of that current will therefore be a measure of the temperature. Such an arrangement is known as a thermo-couple, and is becoming of great use in many manufacturing processes as a means of measuring temperatures.

In the Duddell Thermo-galvanometers, therefore, the alternating current is first led to a "heater" consisting of fine platinised quartz fibre or thin metal wires. Just above the heater there hangs a thermo-couple, consisting of two little bars, one of bismuth and the other of antimony. These two are connected together at their lower end, where they nearly touch the heater, but their upper ends are kept a little apart, being joined, however, by a loop formed of silver strip. This arrangement will be quite clear from the accompanying sketch, and it will be observed that the loop is so shaped that the whole thing can be easily suspended by a delicate fibre which will permit it to swing easily, like the coil in a mirror galvanometer.

It is indeed a swinging coil of a galvanometer formed with a single turn instead of the many turns usual in the ordinary instruments,

and it will be noticed from the sketch that there is a mirror fixed just above the top of the loop.

This coil, then, with the thermo-couple at its lower extremity, is hung between the ends of a powerful magnet much as the fibre of the Einthoven Galvanometer is situated. The alternating current to be measured comes[39] along through the heater. The heater rises in temperature. That warms the lower end of the thermo-couple. Instantly a steady, continuous current begins to circulate round the silver strip which forms the coil, and that, acting just as the current does in the ordinary galvanometer, causes the coil to swing round more or less, which movement is indicated by the spot of light from the mirror. A current as small as twenty micro-amperes (or twenty millionths of an ampere) can be measured in this way.

Mr Duddell has also perfected a wonderful instrument called an Oscillograph, for the strange purpose of making actual pictures of the rise and fall in volume of current in alternating circuits.

Fig. 3.--The "Duddell" Thermo-galvanometer. In this remarkable instrument alternating current enters at a, passes through the fine wire and leaves at b. In doing this it heats the wire, which in turn heats the lower end of the bismuth and antimony bars. This generates continuous current, which circulates through the loop of silver wire, c, which, since it hangs between the poles, d and e, of a magnet, is thereby turned more or less. The amount of the turning indicates the strength of the alternating current.

To realise the almost miraculous delicacy of these wonderful instruments we need first of all to construct a mental picture of what takes place in a circuit through which alternating current is passing. The current begins to flow: it gradually increases in volume until it reaches its maximum: then it begins to die away until it becomes nil: then it begins to grow in the opposite direction, increases to its maximum and dies away once more. That cycle of events occurs over and over again at the rate it may be of hundreds of times

per second. Now for the actual efficient operation of electrical machinery working on alternating current it is very necessary to know exactly how those changes take place--do they[40] occur gradually, the current growing and increasing in volume regularly and steadily, or irregularly in a jumpy manner? Engineers have a great fancy for setting out such changes in the form of diagrams, in which

13

case the alternations are represented by a wavy line, and it is of much importance to obtain an actual diagram showing not what the changes should be according to theory, but what they really are in practice. It is then possible to see whether the "wave-form" of the current is what it ought to be.

Once again we must turn our thoughts back to the string galvanometer. In that case, it will be remembered, there is a conducting fibre passing between the ends or poles of a powerful magnet, the result of which arrangement is that as the current passes through the fibre it is bent by the action of the magnetic forces produced around it. If the current pass one way, downwards let us say, the fibre will be bent one way, while if it pass upwards it will be bent the opposite way. Suppose then that we have two fibres instead of one, and that we send the current up one and down the other. One will be bent inwards and the other outwards. Then suppose that we fix a little mirror to the centre of the fibres, one side of it being attached to one fibre and the other to the other. As one fibre advances and the other recedes the mirror will be turned more or less. Consequently, as the current flowing in the fibres increases or decreases, or changes in direction, the mirror will be slewed round more or less in one direction or the other.

The spot of light thrown by the mirror will then dance from side to side with every variation, and if it be made to fall upon a rapidly moving strip of photograph paper a wavy line will be drawn upon the paper which will faithfully represent the changes in the current.

In its action, of course, it is not unlike an ordinary mirror galvanometer, but its special feature is in the mechanical arrangement of its parts which enable it to[41] move with sufficient rapidity to follow the rapidly succeeding changes which need to be investigated. It is far less sensitive than, say, a Thomson Galvanometer, but the latter could not respond quickly enough for this particular purpose.

[42]

CHAPTER III

THE FUEL OF THE FUTURE

We now enter for a while the realm of organic chemistry, a branch of knowledge which is of supreme interest, since it covers the matters of which our own bodies are constructed, the foods which we eat and the beverages which we drink, besides a host of other things of great value to us.

Although the old division of chemistry into inorganic and organic is still kept up as a matter of convenience, the old boundaries between the two have become largely obliterated. The distinction arose from the fact that there used to be (and are still to a very great extent) a number of highly complex substances the composition of which is known, for they can be analysed, or taken to pieces,

but which the wit of man has failed to put together. Consequently these substances could only be obtained from organic bodies. The living trees, or animals, could in some mysterious way bring these combinations about, but man could not. The molecules of these substances are much more complicated than those with which the inorganic chemist deals. The important ingredient in them all is carbon, which with hydrogen, nitrogen and oxygen almost completes the list of the simple elements of which these marvellous substances are compounded. In some cases there appear to be hundreds of atoms in the molecule.

If one takes a glance at a text-book on organic chemistry the pages are seen to be sprinkled all over with C's and O's, N's and H's, with but an occasional symbol for some other element.

Another feature of this branch which cannot fail to strike[43] the casual observer is the queer names which many of the substances possess. Trimethylaniline, triphenylmethane and mononitrophenol are a few examples which happen to occur to the memory, and they are by no means the longest or queerest-sounding.

Another peculiarity about these organic substances is that a number of them, each quite different from the others, can be formed of the same atoms. Certain atoms of hydrogen, sulphur and oxygen form sulphuric acid, and under whatever conditions they combine they never form anything else. On the other hand, there are sixty-six different substances all formed of eight of carbon, twelve of hydrogen and four of oxygen. This can only mean that in such cases as the latter the atoms have different groupings and that when grouped in one way they form one thing, in another way some other thing, and so on. This explains the extreme difficulty which the chemist finds in building up some of these organic substances.

Every now and again we are startled by some eminent man stating that the time will come when we shall be able to make living things, when the laboratory will turn out living cows and sheep, birds and insects, even man with a mind and soul of his own. Yet one cannot but feel that such men, no matter how great their authority, are simply "pulling the public's leg," to use a colloquial

14

expression. For they hopelessly fail to make many of the commonest things. In many cases where they wish to produce an organic substance they have to call in the aid of some living thing to do it for them, even if it be but a humble microbe. For the microbes perform wonderful feats in chemistry, far surpassing those of the most eminent men. Hence the latter very sensibly use the microbe, employ it to work for them, just set things in order and then stand by while the microbe does the work.

Thus most things can be analysed--that is to say, taken to pieces--while many things can now be synthesised--that is to say, built up from their constituent atoms--but still a[44] great many remain, and among them the most important, the synthesis of which completely baffles man. One of the most useful and widespread substances, for example, cellulose, is, at present at least, utterly beyond us. We do not even know how many atoms there are in the cellulose molecule. The molecules may, for all we know, contain thousands of atoms. Indeed many of these organic matters have very large molecules.

And even if the chemist were able to make all kinds of organic matter, he would still be as far off as ever from making living mat-ter. Indigo used to be derived entirely from plants of that name. One of the greatest triumphs of the organic chemist was when he produced artificial or synthetic indigo. But he is as far off as ever from making the indigo plant. It is claimed that "synthetic" rubber is exactly the same as natural rubber, although some users say it is not quite the same. Still, if it be so, it is dead rubber, not the living part of the plant. The time, then, is infinitely far distant when the chemist will be able to make anything with the characteristics of life--namely, to grow by accretion from within and to reproduce its kind. The most wonderful product of the laboratory is dead. At most it simply resembles something which once was alive.

But that is somewhat of a digression. This dissertation on organic chemistry was simply intended to lead up to the question of liquid fuels, all of which are organic.

In the life of to-day one of the most important things is petroleum. This is a kind of liquid coal. Just how it was formed down in the depths of the earth is not clear. One idea is that it is due to the decomposition of animal and vegetable matter. Another is that certain volcanic rocks which are known to contain carbide of iron might, under the influence of steam, have in bygone ages given off petroleum, or paraffin, to use the other name for the same thing.

In many parts of the world these deposits of oil are obtained by sinking wells and pumping up the oil. In others the[45] liquid gushes out without the necessity of pumping at all. This is believed to be due to the fact that water pressure is at work. Artesian wells, from which the water rushes of its own accord, are quite familiar, and are due to the fact that some underground reservoir tapped by the well is fed through natural pipes, really fissures in the rock, from some point higher than the mouth of the well. Now supposing that

a reservoir of oil were also in communication with the upper world in the same way, the descending water would go to the bottom, underneath the lighter oil, and would thus lift it up, so that on being tapped the oil would rush out.

Another source of mineral oil is shale, such as is to be found in vast deposits in the south-east of Scotland. This shale is mined much as coal is: it is then heated in retorts as coal is heated at the gas-works: and the vapour which is given off, on being condensed, forms a liquid like crude petroleum.

In all these cases the original oil is a mixture of a great number of grades differing from each other in various ways. They are all "hydrocarbons," which means compounds of carbon and hydrogen, and they extend from cymogene (the molecules of which contain four atoms of carbon and ten of hydrogen) to paraffin wax, which has somewhere about thirty-two of carbon to sixty-six of hydrogen. For practical purposes their most important difference is the temperature at which they boil, or turn quickly into vapour.

This forms the means by which they are sorted out. In a huge still, like a steam-boiler, the crude or mixed oil is gradually heated, and the gas given off is led to a cooling vessel where it is chilled back into liquid. The lightest of all, cymogene, is given off even at the freezing-point of water. That is led into one chamber and condensed there. Then, as the temperature rises to 18deg C., rhigolene is given off: that is collected and condensed in another vessel. Between 70deg and 120deg petroleum ether and petroleum naphtha are

produced, and they together constitute what is commonly called petrol. Between 120deg and 150deg petroleum benzine[46] arises. All the foregoing taken together constitute about 8 to 10 per cent. of the whole crude oil. Then between 150deg and 300deg there comes off the great bulk of the oil, nearly 80 per cent., the kerosene or paraffin which we burn in lamps. Above 300deg there is obtained another oil, which is used for lubrication, also the invaluable vaseline, and finally, when the still is allowed to cool, there remains a solid residuum known as paraffin wax. This process is known as fractional distillation, and it will be noticed that it consists essentially in collecting and liquefying separately those vapours which are given off at different ranges of temperature. For our purpose in this chapter we

are mainly concerned with the petrol and the kerosene.

Many efforts have been made in times gone by to use kerosene for firing the boilers of steam-engines. In naval vessels a great deal

is so used at the present time. But the chief method of employing oil for generating power is to use it in an internal combustion-en- gine. These machines have been dealt with at length in Engineering of To-day and Mechanical Inventions of To-day and so must be

15

simply mentioned here. They consist of two types. In one, which is exemplified by the ordinary car or bicycle motor, the oil is gasi-fied in a vessel called a carburetter or vaporiser and then led into the cylinder of the engine, together with the necessary air to enable it to burn. At the right moment a spark ignites the mixture, which burns suddenly, causing a sudden expansion, in other words, an explosion. Thus the power of the engine is derived from a succession of explosions. If the fuel be petrol it vaporises at the ordinary temperature of the engine and needs no added heat. With kerosene, however, heat has to be employed in the vaporiser to make it turn readily into a gas.

The other method is employed in engines of the new "Diesel" type, in which the cylinder of the engine, being already filled with hot air, has a jet of oil sprayed into it. The heat of the air causes it to burst into flame, causing an expansion which drives the engine.

[47]An important feature in the latter type of engine is that the oil is very completely burnt, so that very heavy oils can be used, oils which, if employed in an engine of the other kind, would choke up the cylinder with soot. In other words, the range of oils which can be used in this new kind of engine is much wider than is possible in the others. The latter may be likened to a fastidious man who is very particular about his food, while the former resembles the man of hearty appetite who can eat anything. And just as a man of the latter sort is more easily provided for by the domestic authorities, so the Diesel engine makes the problem of the provision of liquid fuel much simpler.

For it must never be forgotten that the provision of liquid fuel for the world is by no means a simple matter, since the supply is by

no means adequate. The output runs into thousands of millions of gallons, and the whole world is being searched for new fields

of oil, and yet it is all swallowed up as fast as it can be produced, while the coal mines do not feel the competition. A year or so ago the United States and Russia between them (and they are the greatest producers) obtained 5,000,000,000 gallons of oil, seemingly

an enormous quantity. But, on the other hand, Great Britain alone produces over 250,000,000 tons of coal per annum. If, therefore, liquid fuel is to displace coal, as some people lightly think it is going to do, the supply will have to be multiplied many times. In the amount of heat which it is capable of giving the coal of Great Britain alone beats the oil produced by the whole world.

And another thing to be borne in mind is that as the coal miner goes down to the seam and sees for himself what is there, while the oil producer simply stays at the surface and draws it up with a pump, the coal man knows far more as to how much there is still left than the oil man does. We know that the coal deposits will last for many years to come, even if the production go on increasing, whereas the oil supply may fall off in the near future instead of increasing.

[48]And in both cases we are using up capital. Coal is not being made on the earth now, at any rate in any appreciable quantity. The

stage of the earth's history favourable to the formation of coal measures has long gone by. And the same probably applies to oil.

By permission of Dupont Powder Co. Apple Tree Planted with a Spade

This apple tree was planted in the ordinary way with a spade. Compare its size with that in following illustration at p. 48.

It is interesting in this connection to note that coal itself is to a certain extent, or can be at all events, a source of oil. When coal is heated in order to make it give up its gas, or to turn it into coke, vapours are given off which on cooling become coal-tar. At one time regarded only as a crude sort of paint, this is now the source from which many chemical substances are obtained, varying from photographic chemicals to saccharine, a substitute for sugar. So valuable are these products that there is a brisk demand for the tar, in other directions than the manufacture of oils, but oils of various kinds are also obtained from it.

The first step in the operations is fractional distillation, after the manner just described for petroleum. The first "fraction" is "coal-tar naphtha." Then follows "carbolic oil," after that "heavy" or "creosote oil," anthracene oil, and finally there remains in the still on cooling a solid residue known as coal-pitch. The naphtha, on being distilled again, gives, among other things, benzine, from which

the famous aniline dyes are made, and which is useful in many industries. Creosote is largely employed as a preservative for wood, being forced into the timber under high pressure, so that it penetrates right into it and tends to prevent rotting, no matter how wet it may be. Railway sleepers are thus treated, small truck-loads of them being run into a cast-iron tunnel which is then sealed at both ends, while the creosote is forced in by powerful pumps. After such treatment they can lie nearly buried in the damp ballast for a long time without any deterioration.

These coal-tar substances are all very similar to petroleum and its products, hydrocarbons, compounds of hydrogen and carbon in various proportions. Many of them could be used for fuel.

[49]

But since they are based upon the supply of coal, which is itself limited, they cannot, however they may be used, do more than stave off the evil day when the supply will be exhausted.

16

Quite different is it with alcohol, which it seems likely may be the fuel of the future. Some people will be inclined to exclaim "What

a pity to burn it!" since to many the word conveys ideas of another sort altogether. There are many nowadays, however, who, like the writer, have none but a scientific interest in it. To such whisky, for example, is but "impure" alcohol, and it is without the "impurities" that it may become of vast use to the world, thereby possibly repaying man for some of the harm which in the past it has inflicted upon him.

Alcohol, again, is a hydrocarbon. It is really more correct to speak of it in the plural, as "alcohols," since there is a large group of substances all of the same name. Two of these are of the greatest importance, methyl alcohol and ethyl alcohol. The former is obtained from wood, hence it is sometimes called wood spirit. Wood is strongly heated in an iron still, and the methyl alcohol is given off in the form of vapour, which on being collected and cooled condenses into liquid. It is exceedingly unpleasant to the taste: if it were the only kind there would be no consumption of alcohol as a drink.

The second kind mentioned is obtained by the agency of germs or microbes, and the story of its production is so interesting as to demand a little space.

We will commence with the maltster. He performs the first part of the operation. Starting with ordinary barley, by the action of heat,

aided by natural growth, he produces the raw material on which the brewer may work. Now barley, like all grain, is largely made up of starch, and although starch will not make alcohol, it can be turned into sugar, which will. So the task of the maltster is to commence the change of the starch in the grain into sugar.

First of all it is soaked in water and spread upon floors[50] and heated until it begins to sprout. There is a little part in each grain called the endosperm, which is the embryonic plant, and the starch is really the food provided by nature to nourish the growing endosperm until such time as it shall be strong enough to draw its nourishment from the soil. In order that it may not be washed away prematurely, the starch is locked up by nature in closely fastened cells, and, moreover, it is insoluble, so that water cannot carry it away. The endosperm, however, has at its disposal certain substances known as enzymes (and it increases its store of these as

it grows), one of which is able to dissolve away the walls of the cells, to unlock the treasures, as it were, while the other turns the insoluble starch into soluble matter, in which state the growing organism is able to make use of it as food.

So as the grain sprouts upon the maltster's floor this process is going on--the cells are being opened and their contents converted from starch into soluble matters. Then, when the growth has gone far enough, the grain is transferred to a kiln, where it is subjected to heat, by which the growth is stopped. The living part of the grain is, in fact, killed. That is mainly to stop the young plant from eating up the altered starch, which it would do if allowed time, but which the brewer wants to be kept for his own use.

The maltster's task is now finished, and we come to the brewer's. The first thing he does with the malt is to crush it between rolls, thereby liberating thoroughly those substances which have been formed from the starch and which he intends to turn into sugar. Having crushed it, he places it in the "mash tun," a large tank of wood or iron, in which it is mixed with water and subjected to heat. While in this vessel the enzymes become active again and turn the soluble starch, or a part of it, into a kind of sugar.

The liquid drawn off from the mash tun, containing, of course, the sugar, is subsequently boiled, numerous flavouring matters (in-

cluding hops) are added, and then it is cooled again, ready for the final process--fermentation.

[51]This takes place in a large vat or "tun" and is brought about by the agency of yeast which is added to the liquid.

Now yeast is a multitude of microscopic plants round in shape and about one three-thousandth of an inch in diameter. Though so small, this little organism is really quite complicated in its structure, and within its little body there are carried on complicated chemical changes which baffle entirely the most learned chemist to imitate. Further, he has yet to find out how the little yeast plant does it. He not only cannot imitate the process, he does not know what the process is. These little organisms multiply mainly by the process of "budding." A new one grows out of the side of each old one, rapidly reaches maturity, breaks away and commences an independent existence. No sooner is it free than it in turn gives birth to another. Indeed so great is its hurry to propagate itself that sometimes the new cell begins to throw out a bud before it has itself separated from its parent. It is therefore easy to see that yeast increases in quantity by what some call "leaps and bounds," but which the mathematically minded know as geometrical progression.

The particular form of sugar with which we are concerned here is known as "dextro-glucose." This the yeast splits up into alcohol and carbonic acid gas. The latter bubbles up to the surface, and escapes into the air, while the alcohol becomes dissolved in the wa-tery liquid. It is believed that the yeast performs this operation not directly, but by the production of certain enzymes, which in their turn act upon the sugar.

17

The liquid so formed is beer. But since it is alcohol with which we are concerned, and not beer, many details connected with its manufacture have been omitted. Enough has been said, however, to show that by comparatively simple processes grain of all sorts, in fact, anything which contains starch, and such things are to be found in worldwide profusion, can be turned into alcohol. All the

really intricate chemical functions are performed readily and[52] cheaply by living organisms. All man has to do is to set up the conditions under which the organisms can work.

In the process just described only a portion of the starch in the grain is converted into sugar, hence the percentage of alcohol in beer is comparatively small. If all the starch be converted a liquid much stronger in alcohol is produced, and if that be distilled, so as to separate the spirit from the water with which it is mixed, there results whisky. Brandy, likewise, is the spirit distilled from wine,

rum from molasses, and so on. In all these familiar beverages the essential feature is this same alcohol, of the variety known as ethyl alcohol.

It will be noticed that in the making of beer the alcohol is actually formed in water. There is a sugary water which under the action of the yeast becomes an alcoholic water. And this indicates a very useful feature about the liquid when used for industrial purposes. A tank full of petrol is extremely dangerous, so much so that the storage of petrol is hedged about by all manner of precautions. The danger is that it gives off an inflammable vapour and that if it once begin to burn there is practically no possibility of put-

ting it out. Being lighter than water, it simply clothes with a layer of fire any water which may be thrown on to it. The water in such circumstances simply serves to spread the naming petrol about and so to make matters worse. Now alcohol, with its partiality for the companionship of water, behaves quite differently. True, it also may give off an inflammable vapour, but if a quantity of it catch fire it can be extinguished in the usual way by a fire-engine. The water and alcohol immediately combine--the alcohol becomes dissolved in the water just as sugar may do, and as soon as the percentage of water in the mixture becomes considerable the burning stops.

It may be that some readers will have discovered this fact for themselves without knowing precisely what it was. It is a common dodge with amateur photographers if they want to dry a negative quickly to immerse it in methylated[53] spirit. The spirit seems to take the water out of the film and, itself drying quickly, leaves the negative in a perfectly dry condition in a few minutes. Now after using spirit in that way it is useless to put it in a spirit stove or lamp. It will not burn. Methylated spirit is alcohol, and the reason why it has such a quick drying action is that it and the water in the wet film quickly mix. After immersion the film is wet, not with water merely, but with a mixture of a lot of spirit and a little water. Hence the speed with which it evaporates. And the non-inflammability of the mixture is due to the presence of the water.

Methylated spirit only differs from the alcohol in alcoholic beverages in that something is added to make it undrinkable. Owing to the craving for it, which is so widespread, and the doubtful effect which it has on certain citizens, most states regard it as pre-emi- nently a subject for taxation, thereby on the one hand bringing in a good revenue, and on the other discouraging its too free use. But those considerations apply only to drinkable alcohol. That which is to be used for industrial purposes is not in any way a legitimate object for taxation. Hence the problem arises of making a form of alcohol which shall answer all the needs of the industries which use it, and at the same time be so repulsive to the senses that no one can possibly drink it. This result is achieved by adding some of the methyl alcohol derived from the vapour given off by wood when heated. Commonly known as "wood spirit," this is so unpleasant that it renders the mixture of no use for drinking, and so it can safely be freed from taxation.

Unfortunately this spirit has less heating value than petrol. That means that a given quantity of each liquid will produce more heat in the case of petrol than in the case of alcohol. Indeed the difference is about two to one. Hence an engine to give out a certain horse-power would need to have its cylinders twice as big if it were to use alcohol instead of the other fuel. There is a certain compensa-tion, however, in the fact that alcohol is very easily compressible.[54] In modern internal combustion-engines much of the efficiency is due to the explosive charge which is drawn into the cylinder being compressed into a small space before it is fired. It was the discovery of the value of compressing the gas which made the gas-engine so formidable a rival to the steam-engine, and the wonderful performances of the Diesel engines are due very largely to the fact that the air is compressed in the cylinder to a very high pressure. The jet of oil burns in highly compressed air. And because of the facility with which alcohol can be compressed it is said to be more effective as a source of motive power than would be expected from its comparatively feeble heat.

Thus we may sum up the possibilities of the future. Coal, petroleum and their derivatives exist in limited quantities in the world, and so far as we can see the vast drafts which we are taking from them are not being replaced, indeed at this stage of the earth's development cannot be replaced, by any more. Sooner or later we must come to an end of them. Is it not comforting, therefore, to know that there is another source of fuel at hand, inexhaustible, since it can be produced as needed. We have only to set the sun and the ground to work to produce grain, rice, potatoes, or any of the myriad substances which contain starch, and from that, by simple and well-known processes, we can obtain a cheap, safe and reliable fuel. Indeed there seems nothing but the ultimate loss of sunlight, countless millions of years hence, which can ever check the supply of this valuable commodity. What has doubtless, in many cases, been a curse in the past may turn out to be the great boon of the future.

18

[55] CHAPTER IV

SOME VALUABLE ELECTRICAL PROCESSES

Students of that branch of science known as physics are coming to the conclusion that electricity plays a much more important part in the universe than was supposed. They are led to believe that electrical attraction is the cement which binds together those exceedingly minute particles out of which everything is built up. Whether electricity binds them together or not, it is certain that electrical action can in some cases separate those particles, and this process of separation provides a means of carrying on some very remarkable and useful industrial processes.

Let us imagine a vessel filled with water to which has been added a little sulphuric acid, while suspended in it are two strips of platinum. There is a space between the strips, so that when their upper ends are suitably connected to a source of electric current that current flows from one strip to the other through the liquid.

That is an example of the apparatus for carrying out this electrical separation in its simplest form, and it will facilitate the further description if the names of various parts are enumerated.

The process itself is electrolysis; the liquid is the electrolyte, while the strips are the electrodes. The individual electrodes, again, have special names, that by which the current enters being the anode and that by which it leaves the cathode. It is not difficult to remember which is which if we bear in mind that the current traverses them in alphabetical order. Since, however, it may not be easy for the general reader to carry all these terms in his mind, we will,[56] when it is necessary to differentiate between the two electrodes, call one the in-electrode and the other the out-electrode.

Returning now to our imaginary apparatus, let us turn on the current. At first nothing seems to be happening, although suitable instruments would show that current was flowing. Soon, however, little bubbles appear upon the electrodes, and these grow larger and larger, until they detach themselves from the platinum to which they have been adhering, float up to the surface and burst. The question which naturally arises is, What do those bubbles consist of ? Are they air?

If we take means to collect the gases which formed them we get an unmistakable answer. The bubbles which arise from the in-electrode are oxygen, those from the other hydrogen. If we allow our apparatus to work for some time, and collect all the gas which arises, we shall find that there is twice as much hydrogen as oxygen. We shall also find, as the process goes on, that the quantity of water diminishes.

Perhaps I may be allowed at this point to remind my readers that water is a collection of minute ultra-microscopic particles called "molecules," each of which is formed of three smaller particles still called "atoms." Of the three atoms two are hydrogen and one oxygen. Water therefore consists of hydrogen and oxygen, there being twice as much of the former as there is of the latter.

We see, therefore, that electrolysis gives us hydrogen and oxygen in exactly those proportions in which they occur in water, and since we also see that as these gases appear the water itself disappears, we are led to conclude that the current is splitting up the water into the gases of which it is formed.

But the strange thing is that this will not work with pure water. We have to add something to it. In the case of our imaginary experiment it was sulphuric acid. What part does that play?

[57]This is not fully understood, but we may be able to form a mental picture of what is believed to happen as follows.

The in-electrode is surrounded by a vast assemblage of these tiny molecules, most of them those of water, but a few those of the acid. The latter are more complex in their structure than the former, but they too contain hydrogen. Current flows into the electrode and instantly hydrogen atoms from the acid molecules crowd round it, like boatmen at the seaside anxious to secure a passenger. Each takes on board a quantity of electricity and with it darts across the intervening space to the other electrode. Arrived there, it

gives up its load and, its work done, remains lying upon the electrode until enough others like unto itself have gathered there to form a bubble and so escape. These hydrogen atoms are thought to be the craft which carry the current through the liquid and enable it to pose, as it were, as a conductor of electricity, which in reality it is not.

But where does the oxygen come from?

19

To find the answer to that we must add a second chapter to our story. When the hydrogen "boats" took on board their load of electricity they left their former associates, and these forthwith "set upon" neighbouring water molecules and with the audacity of high-waymen stole from them enough hydrogen atoms to take the place of those they had lost. Thus the acid molecules became complete once more, while the scene of the conflict near the in-electrode was strewn with the remains of the water molecules from which the hydrogen atoms had been stolen. These remains, of course, would be oxygen, and they, collecting together on the electrode, would eventually be in numbers sufficient to form bubbles and so escape.

Thus it may be the acid which really does the work, yet because of its subsequent raid upon the water it is the latter which disappears, and it is the materials of the latter which are bought to the surface in the bubbles.

And there we see the mechanism whereby, so it is believed, electric current can pass through otherwise non-conducting[58] liquids. And the important point, as far as practical utility is concerned, is that the passage of the current is accompanied by a splitting up of something or other, either the water or something in it, the materials of which are deposited, one on one electrode and the other on the other.

And now we can proceed to those useful applications of electrolysis, the commonest of which, perhaps, is electro-plating.

We have seen how electrolysis causes hydrogen, probably out of the acid, to be deposited upon one electrode. Suppose that, instead of an acid, we put in the water one of those substances known to chemists as a "salt," the commonest example of which is ordinary table salt. This well-known condiment is caused by the interaction of hydrochloric acid and the metal sodium and will serve to illustrate what all salts are.

All acids are compounds of hydrogen and something else, and their biting action is due to the readiness with which the "something else" evicts the hydrogen and takes in a metal in its place. Thus hydrochloric acid, given the opportunity, gets rid of its hydrogen and takes in sodium, thereby forming chloride of soda or common salt.

Another example is the gold chloride familiar to photographers. This is the result of the action of certain acids upon gold, wherein the acids throw out their hydrogen and take in gold instead.

To sum up, then, a salt is just the same sort of thing as an acid, like the sulphuric acid which we used in our "experiment," except that some metal has taken the place of the hydrogen.

It is not surprising, then, to find that if we put a salt in the electrolyte instead of an acid we get a similar result. In the one case hydrogen is deposited upon the out-electrode, in the other the metal. In the former case, since hydrogen is a gas, it forms bubbles and floats away, but in the latter the solid metal remains a thin, even coating upon the electrode. That is the principle of electro-plating.

[59]The electrolyte consists of a suitable solution containing a salt of the metal to be deposited, and it is placed in an insulating ves-sel or vat. The articles to be plated form the out-electrode, so that they have to be suspended in some convenient way from a metal conductor by conducting wires. Of course they are entirely immersed in the liquid. The in-electrode is sometimes a plate of platinum (the reason that expensive metal is used being that it is unaffected by the chemicals) or else a plate of the metal being deposited.

In the former case, the solution becomes weaker as the work proceeds, and more salt has to be added. In the latter, however, the strength of the solution remains unchanged, for by an interesting interchange the in-electrode adds to it just what it loses by deposition upon the other one. The effect is therefore just as if the current tore off particles from the one and placed them upon the other.

This is believed to be due to the agency of the oxygen which in the case of the electrolysis of water becomes free, but which in this case forms with the metal electrode a layer of oxide upon its surface, this oxide being then dissolved away by the liquid. Thus as fast as the metal is deposited upon the out-electrode its place is taken by more metal from the in-electrode.

In some processes it is desired to deposit metal upon a non-conducting surface, and it is evident that such cannot be used as an electrode. Nor is it any use to attempt to deposit upon anything except an electrode. The only thing to do, then, is to make the object a conductor by some means. Models in clay, wax and plaster, once-living objects like small animals, fruit, flowers or insects, can, however, have a perfect replica made of them by electrical deposition, by the simple method of coating the surface to be plated with a thin layer of plumbago. This skin, although extremely thin, is a sufficiently good conductor to make the process possible. Process blocks for printing are copied in this way, so that a particularly delicate example of the blockmaker's art need not be worn down by much pressing,[60] copies or "electros" being made off it for actual use in the press.

The original block is a plate of copper on which the picture is represented by minute depressions and prominences. On this a layer of soft wax is pressed, so as to obtain a perfect but reversed copy. Having been coated with plumbago, this is then put into a vat

20

containing a solution of copper salts and is used as the out-electrode, the other being a plate of copper. When the current is turned on the copper is thus deposited on the wax until a thin sheet of copper is formed which is an exact but reversed copy of the wax, a direct copy, that is, of the original block.

The back of this thin sheet is then covered with molten lead or type metal to fill up any depressions and to give it sufficient strength.

Anyone who has seen one of these "half-tone" blocks covered with minute depressions so slight that they can scarcely be seen, yet so perfect that a beautiful print can be obtained from them, will realise the wonderful power of this electrolytic process, the marvellous accuracy with which the original is copied, and the unerring way in which the electric current carries the particles of copper into every one of the myriad recesses in the wax.

Another specimen of the marvellous work of this system is the wax cylinder of the phonograph. The sound is produced by a needle trailing along a groove of varying depth cut in the surface of the cylinder. This groove forms a spiral, passing round and round like the thread of a screw, and it encircles the cylinder one hundred times in every inch of its length. Consequently, at any point one may take, there is but one one-hundredth of an inch from the centre of one turn to the centre of the turn on either side of it. And at its deepest the groove is less than one-thousandth of an inch deep. The phonograph itself cuts the first "master" record, as it is termed, and the problem is to take a number of casts off this model of such delicacy and accuracy that every variation in that exceedingly

fine groove shall be faithfully reproduced. Such a task might well be given up[61] as hopeless, but with the help of electrolysis it is

accomplished easily and cheaply.

To attempt to press anything upon the surface of the "master" would but smooth out the soft wax and obliterate the groove altogether. To apply anything softened by heating would be to melt it. But electrolysis, without tending in any way to distort or damage the delicately cut surface, deposits upon it a surface of metal from which thousands of casts can be made. The gentle fingers of the electricity overlay the soft wax with the hard, strong metal with a minute perfection almost beyond belief.

To commence with, the master record is placed upon a sort of turntable in a vacuum and turned round in the neighbourhood of two strips of gold-leaf strongly electrified. By this means the gold is vaporised and a perfect coating of gold is laid upon the wax. This is far too thin to be of any use, except to render the cylinder a conductor, for the coating is so fragile that it is no stronger than the wax itself. It enables the cylinder, however, to be electro-plated with copper until it is surrounded by a strong metallic shell a sixteenth of an inch thick. It takes about four days to deposit this thickness. The copper shell is then turned smooth in a lathe and fitted tightly into a brass jacket. A little cooling causes the wax record to shrink sufficiently to free it from the copper shell and allow it to be lifted out. A copper mould is thus formed in which any number of additional records can be cast. The molten wax is simply introduced into the inside, and allowed to set; the inside is bored out in a lathe, and then with a little cooling it shrinks and can be withdrawn, a completely finished record, every tiny depression or swelling in the original master being reproduced with an accuracy almost incredible.

Another valuable use to which this process is put is the purification of metals. The electro-chemical action works with unerring precision: it never mistakes an atom of iron for an atom of copper, for example. Passing through a solution of copper salt, the current deposits only copper.

[62]For modern electrical machinery and apparatus copper is required of the utmost possible purity, for every impurity adds to its electrical resistance, in other words, diminishes its value as a conductor. Consequently thousands of tons of "electrolytic" copper, as it is termed, are produced every year. The electrodes used are plates of ordinary copper. A coating of pure metal is deposited

by electrolysis upon the out-electrode from the other one. When the deposit is thick enough the out-electrode is taken out and the deposit torn off it, the union between the two being sufficiently imperfect for this to be done without difficulty. The metal of which the in-electrode is made has already been purified by other processes, until it contains but one per cent. of foreign matter, and by this means even that small percentage is entirely got rid of. The impurities fall to the bottom of the vessel in the form of "slime," which

is periodically removed.

And not only is electrolysis thus unerring in picking out certain atoms from among a mixture, but there is an exact relation between the work done and the quantity of current used. Consequently it forms a very exact method of measuring currents. The method

of measuring current by the strength of the magnetic field which it produces has been mentioned already, and such measurements can be checked by electrolysis. Thus the practical definition of the ampere is "that current which when passed through a solution of silver nitrate in water will deposit silver at the rate of *001118 gramme per second."

The electric accumulator or secondary battery, one of the most useful appliances, is the result of electrolysis reversed. Many large electric-lighting plants have in addition to their generating machinery a large battery of secondary cells, which, being kept charged, are able to help the machinery in times of heavy demand, or even to supply the whole current needed for, say, half-an-hour, so that the whole of the machinery could, in the event of an accident, be shut down for that time and the supply maintained from the bat-

21

teries.[63] This would be sufficient in many cases for fresh machinery to be brought into action or emergency arrangements to be

made.

It may be that this book is being read by someone seated serenely in his arm-chair while engineers and workmen at the generating station are working in frantic haste to set right some sudden breakdown before the batteries are run down. The batteries may have saved the town half-an-hour's darkness.

Large telegraph offices are fitted with secondary batteries. Many motorists owe the ignition which keeps their engines at work to secondary batteries. It is secondary batteries which keep the wireless apparatus at work on a wrecked vessel after the engines have stopped. Indeed secondary batteries are one of the most beneficent inventions. And if only they could be made in a lighter form than is possible at present their value would be infinitely increased.

We have seen how the passage of current through acidulated water produces hydrogen and oxygen. If those gases be collected in closed vessels over the water, so that they remain in contact with the water, as soon as the current is stopped a reverse action sets in. The gases tend to recombine with the electrolyte and in so doing to give back a current equal to that which formed them. Fig.

4 shows the construction of what is called a voltameter, in which the gases arising from the electrodes are collected in little glass vessels placed just above them. Such an apparatus enables us to see easily how the accumulator works. The picture shows the battery which is effecting the separation of the oxygen and hydrogen. If that be disconnected, and the wires joined, as shown by the dotted line, a current will flow back until the oxygen and hydrogen have returned into the solution again. The apparatus will, in fact, work like an ordinary battery, except that instead of a plate or rod of zinc a mass of hydrogen will form the essential part.

An appliance such as a voltameter is not of much use for the practical purpose of storing large quantities of electrical[64] energy, because the surfaces of the electrodes are so small and the surfaces where liquid and gases are in contact are small too. It is clear that the larger the electrodes are the wider will be the passage for the current, just as a wide road can accommodate more traffic than a narrow path. We may regard the electrodes as like gateways through which the current passes. By making them large, therefore, we enable a large current to pass, and consequently permit electrolysis to take place with great comparative rapidity.

Fig. 4

The "plates," as the electrodes in a secondary battery are termed, are generally large metal plates. Experiment has shown that lead is the best for this purpose. It has also been found that it can be improved by making it porous, since the inner surfaces of the pores are so much added surface through which current can pass into the electrolyte. There are various ways of producing this porosity, which need not trouble us here, however. It will suffice for our purpose to understand that an ordinary secondary cell consists of two lead plates, with the largest possible surface, immersed in a liquid, generally a dilute solution of sulphuric acid in water.

To charge the battery, current is sent to one plate, through the liquid to the other plate, and so away. A thin film of hydrogen is thus

formed upon the outgoing plate, while oxygen is formed at the incoming one. Since the hydrogen is spread over such a large area, it does not accumulate[65] sufficiently for much of it to rise to the surface. Most of it remains adhering to the plate. The oxygen combines with the lead of its plate and so is safely stored up there in the form of oxide of lead. This storage of hydrogen upon the one plate and oxygen on the other cannot go on indefinitely, and so as soon as the limit is reached the cell is fully charged. Passage of further current is then simply waste.

The dynamo or primary batteries which are used for charging having been disconnected, the two plates can be connected together through lamps, motors, or in any other desired way, and the current will then flow out again, the opposite way to that in which it entered, just as a stone thrown up in the air returns the opposite way. The current which comes out is, in fact, a sort of reflex action arising from that which went in, the mechanism by which it is produced being the reabsorption of the oxygen and hydrogen into the electrolyte.