Читать книгу Electric Bells and All About Them: A Practical Book for Practical Men - Selimo Romeo Bottone - Страница 9

Table of Contents

§ 17. If we immerse a strip of ordinary commercial sheet zinc in dilute acid (say sulphuric acid 1 part by measure, water 16 parts by measure[4]), we shall find that the zinc is immediately acted on by the acid, being rapidly corroded and dissolved, while at the same time a quantity of bubbles of gas are seen to collect around, and finally to be evolved at the surface of the fluid in contact with the plate. Accompanying this chemical action, and varying in a degree proportionate to the intensity of the action of the acid on the zinc, we find a marked development of heat and electricity. If, while the bubbling due to the extrication of gas be still proceeding, we immerse in the same vessel a strip of silver, or copper, or a rod of graphite, taking care that contact does not take place between the two elements, no perceptible change takes place in the condition of things; but if we cause the two strips to touch, either by inclining the upper extremities so as to bring them in contact out of the fluid like a letter Λ, or by connecting the upper extremities together by means of a piece of wire (or other conductor of electricity), or by causing their lower extremities in the fluid to touch, we notice a very peculiar change. The extrication of bubbles around the zinc strip ceases entirely or almost entirely, while the other strip (silver, copper, or graphite) becomes immediately the seat of the evolution of the gaseous bubbles. Had these experiments been performed with chemically pure metallic zinc, instead of the ordinary impure commercial metal, we should have found some noteworthy differences in behaviour. In the first place, the zinc would have been absolutely unattacked by the acid before the immersion of the other strip; and, secondly, all evolution of gas would entirely cease when contact between the two strips was broken.

As the property which zinc possesses of causing the extrication of gas (under the above circumstances) has a considerable influence on the efficiency of a battery, it is well to understand thoroughly what chemical action takes place which gives rise to this evolution of gas.

§ 18. All acids may be conveniently regarded as being built up of two essential portions, viz.: firstly, a strongly electro-negative portion, which may either be a single body, such as chlorine, iodine, bromine, etc., or a compound radical, such as cyanogen; secondly, the strongly electro-positive body hydrogen.

Representing, for brevity's sake, hydrogen by the letter H., and chlorine, bromine, iodine, etc., respectively by Cl., Br., and I., the constitution of the acids derived from these bodies may be conveniently represented by:—

H Cl	H Br	H I
┗━━┛	┗━━┛	┗━━┛
Hydrochloric Acid[5].	Hydrobromic Acid.	Hydriodic Acid.

and the more complex acids, in which the electro-negative component is a compound, such as sulphuric acid (built up of 1 atom of sulphur and 4 atoms of oxygen, united to 2 atoms of hydrogen) or nitric acid (consisting of 1 nitrogen atom, 6 oxygen atoms, and 1 hydrogen atom), may advantageously be retained in memory by the aid of the abbreviations:—

H2SO4		HNO6
┗━━━┛		┗━━┛
Sulphuric Acid[6].	and	Nitric Acid[7].

When zinc does act on an acid, it displaces the hydrogen contained in it, and takes its place; the acid losing at the same time its characteristic sourness and corrosiveness, becoming, as chemists say, neutralized. One atom of zinc can replace two atoms of hydrogen, so that one atom of zinc can replace the hydrogen in two equivalents of such acids as contain only one atom of hydrogen.

This power of displacement and replacement possessed by zinc is not peculiar to this metal, but is possessed also by many other bodies, and is of very common occurrence in chemistry; and may be roughly likened to the substitution of a new brick for an old one in a building, or one girder for another in an arch.

It will be well, therefore, to remember that in all batteries in which acids are used to excite electricity by their behaviour along with zinc, the following chemical action will also take place, according to which acid is employed:—

Hydrochloric Acid	and	Zinc,	equal	Zinc Chloride	and	Hydrogen Gas.
2HCl	+	Zn	=	ZnCl2	+	H2

or:—

Sulphuric Acid	and	Zinc,	equal	Zinc Sulphate	and	Hydrogen Gas.
H2SO4	+	Zn	=	ZnSO4	+	H2

Or we may put this statement into a general form, covering all cases in which zinc is acted on by a compound body containing hydrogen, representing the other or electro-negative portion of the compound by X:—

Zn + H₂X = ZnX + H₂

the final result being in every case the corrosion and solution of the zinc, and the extrication of the hydrogen gas displaced.

§ 19. We learn from the preceding statements that no electricity can be manifested in a battery or cell (as such a combination of zinc acid and metal is called) without consumption of zinc. On the contrary, we may safely say that the more rapidly the useful consumption of zinc takes place, the greater will be the electrical effects produced. But here it must be borne in mind that if the zinc is being consumed when we are not using the cell or battery, that consumption is sheer waste, quite as much as if we were compelled to burn fuel in an engine whether the latter were doing work or not. For this reason the use of commercial zinc, in its ordinary condition, is not advisable in batteries in which acids are employed, since the zinc is consumed in such, whether the battery is called upon to do electrical work (by placing its plates in connection through some conducting circuit) or not. This serious objection to the employment of commercial zinc could be overcome by the employment of chemically purified zinc, were it not that the price of this latter is so elevated as practically to preclude its use for this purpose. Fortunately, it is possible to confer, on the ordinary crude zinc of commerce, the power of resisting the attacks of the acid (so long as the plates are not metallically connected; or, in other words, so long as the "circuit is broken"), by causing it to absorb superficially a certain amount of mercury (quicksilver). The modes of doing this, which is technically known as amalgamating the zinc, are various, and, as it is an operation which every one who has the care of batteries is frequently called upon to perform, the following working details will be found useful:—

§ 20. To amalgamate zinc, it should first be washed with a strong solution of common washing soda, to remove grease, then rinsed in running water; the zinc plates, or rods, should then be dipped into a vessel containing acidulated water (§ 17), and as soon as bubbles of hydrogen gas begin to be evolved, transferred to a large flat dish containing water. While here, a few drops of mercury are poured on each plate, and caused to spread quickly over the surface of the zinc by rubbing briskly with an old nail-brush or tooth-brush. Some operators use a kind of mop, made of pieces of rag tied on the end of a stick, and there is no objection to this; others recommend the use of the fingers for rubbing in the mercury. This latter plan, especially if many plates have to be done, is very objectionable: firstly, on the ground of health, since the mercury is slowly but surely absorbed by the system, giving rise to salivation, etc.; and, secondly, because any jewellery, etc., worn by the wearer will be whitened and rendered brittle. When the entire surface of the zinc becomes resplendent like a looking-glass, the rubbing may cease, and the zinc plate be reared up on edge, to allow the superfluous mercury to drain off. This should be collected for future operations. It is important that the mercury used for this purpose should be pure. Much commercial mercury contains lead and tin. These metals can be removed by allowing the mercury to stand for some time in a vessel containing dilute nitric acid, occasional agitation being resorted to, in order to bring the acid into general contact with the mercury. All waste mercury, drainings, brushings from old plates, etc., should be thus treated with nitric acid, and finally kept covered with water. Sprague, in his admirable work on electricity, says:—"Whenever the zinc shows a grey granular surface (or rather before this), brush it well and re-amalgamate, remembering that a saving of mercury is no economy, and a free use of it no waste; for it may all be recovered with a little care. Keep a convenient sized jar, or vessel, solely for washing zinc in, and brush into this the dirty grey powder which forms, and is an amalgam of mercury with zinc, lead, tin, etc., and forms roughnesses which reduce the protection of the amalgamation. Rolled sheet zinc should always be used in preference to cast. This latter is very hard to amalgamate, and has less electro-motive power[8]; but for rods for use in porous jars, and particularly with saline solutions, cast-zinc is very commonly used. In this case great care should be taken to use good zinc cuttings, removing any parts with solder on them, and using a little nitre as a flux, which will remove a portion of the foreign metals."

§ 21. Another and very convenient mode of amalgamating zinc, specially useful where solid rods or masses of zinc are to be used, consists in weighing up the zinc and setting aside four parts of mercury (by weight) for every hundred of the zinc thus weighed up. The zinc should then be melted in a ladle, with a little tallow or resin over the top as a flux. As soon as melted, the mercury should be added in and the mixture stirred with a stick. It should then be poured into moulds of the desired shape. This is, perhaps, the best mode of amalgamating cast zincs.

§ 22. Some operators recommend the use of mercurial salts (such as mercury nitrate, etc.) as advantageous for amalgamating; but, apart from the fact that these salts are generally sold at a higher rate than the mercury itself, the amalgamation resulting, unless a very considerable time be allowed for the mercuric salts to act, is neither so deep nor so satisfactory as in the case of mercury alone. It may here be noted, that although the effect of mercury in protecting the zinc is very marked in those batteries in which acids are used as the exciting fluids, yet this action is not so observable in the cases in which solutions of salts are used as exciters; and in a few, such as the Daniell cell and its congeners, the use of amalgamated zinc is positively a disadvantage.

§ 23. If, having thus amalgamated the zinc plate of the little battery described and figured at § 9, we repeat the experiment therein illustrated, namely, of joining the wires proceeding from the two plates over a suspended magnetic needle, and leave them so united, we shall find that the magnetic needle, which was originally very much deflected out of the line of the magnetic meridian (north and south), will very quickly return near to its old and normal position; and this will be found to take place long before the zinc has been all consumed, or the acid all neutralised. Of course, this points to a rapid falling off in the transmission of the electric disturbance along the united wires; for had that continued of the same intensity, the deflection of the needle would evidently have remained the same likewise. What, then, can have caused this rapid loss of power? On examining (without removing from the fluid) the surface of the copper plate, we shall find that it is literally covered with a coating of small bubbles of hydrogen gas, and, if we agitate the liquid or the plates, many of them will rise to the surface, while the magnetic needle will at the same time give a larger deflection. If we entirely remove the plates from the acid fluid, and brush over the surface of the copper plate with a feather or small pledget of cotton wool fastened to a stick, we shall find, on again immersing the plates in the acid, that the effect on the needle is almost, if not quite, as great as at first; thus proving that the sudden loss of electrical energy was greatly due to the adhesion of the free hydrogen gas to the copper plate. This peculiar phenomenon, which is generally spoken of as the polarisation of the negative plate, acts in a twofold manner towards checking the electrical energy of the battery. In the first place, the layer of hydrogen (being a bad conductor of electricity) presents a great resistance to the transmission of electrical energy from the zinc plate where it is set up to the copper (or other) plate whence it is transmitted to the wires, or electrodes. Again, the copper or other receiving plate, in order that the electric energy should be duly received and transmitted, should be more electro-negative than the zinc plate; but the hydrogen gas which is evolved, and which thus adheres to the negative plate, is actually very highly electro-positive, and thus renders the copper plate incapable of receiving or transmitting the electric disturbance. This state of things may be roughly likened to that of two exactly equal and level tanks, Z and C, connected by a straight piece of tubing. If Z be full and C have an outlet, it is very evident that Z can and will discharge itself into C until exhausted; but if C be allowed to fill up to the same level as Z, then no farther flow can take place between the two.

It is, therefore, very evident that to ensure anything like constancy in the working of a battery, at least until all the zinc be consumed or all the acid exhausted, some device for removing the liberated hydrogen must be put into practice. The following are some of the means that have been adopted by practical men:—

§ 24. Roughening the surface of the negative plate, which renders the escape of the hydrogen gas easier. This mode was adopted by Smee in the battery which bears his name. It consists of a sheet of silver, placed between two plates of zinc, standing in a cell containing dilute sulphuric acid, as shown at Fig. 5.

Fig. 5.

The silver sheet, before being placed in position, is platinised; that is to say, its surface is covered (by electro-deposition) with a coating of platinum, in the form of a fine black powder. This presents innumerable points of escape for the hydrogen gas; and for this reason this battery falls off much less rapidly than the plain zinc and smooth copper form. A modification of Smee's battery which, owing to the large negative surface presented, is very advantageous, is Walker's graphite cell. In this we have a plate of zinc between two plates of gas-carbon ("scurf"), or graphite. The surface of this body is naturally much rougher than metal sheets; and this roughness of surface is further assisted by coating the surface with platinum, as in the case of the Smee. The chief objection to the use of graphite is its porosity, which causes it to suck up the acid fluid in which the plates stand, and this, of course, corrodes the brass connections, or binding screws.

Other mechanical means of removing the hydrogen have been suggested, such as brushing the surface of the plate, keeping the liquid in a state of agitation by boiling or siphoning; but the only really efficient practical means with which we are at present acquainted are chemical means. Thus, if we can have present at the negative plate some substance which is greedy of hydrogen, and which shall absorb it or combine with it, we shall evidently have solved the problem. This was first effected by Professor Daniell; and the battery known by his name still retains its position as one of the simplest and best of the "constant" forms of battery. The term "constant," as applied to batteries, does not mean that the battery is a constancy, and will run for ever, but simply that so long as there is in the battery any fuel (zinc, acid, etc.), the electrical output of that battery will be constant. The Daniell cell consists essentially in a rod or plate of zinc immersed in dilute sulphuric acid, and separated from the copper or collecting plate by a porous earthen pot or cell. Around the porous cell, and in contact with the copper plate, is placed a solution of sulphate of copper, which is maintained saturate by keeping crystals of sulphate of copper (blue stone, blue vitriol) in the solution. Sulphate of copper is a compound built up of copper Cu, and of sulphur oxide SO₄. When the dilute sulphuric acid acts on the zinc plate or rod (§ 18), sulphate of zinc is formed, which dissolves in the water, and hydrogen is given off:—

Zn	+	H2SO4	=	ZnSO4	+	H2.
Zinc	and	sulphuric acid	produce	zinc sulphate	and	free hydrogen.

Now this free hydrogen, by a series of molecular interchanges, is carried along until it passes through the porous cell, and finds itself in contact with the solution of copper sulphate. Here, as the hydrogen has a greater affinity for, or is more greedy of, the sulphur oxide, SO₄, than the copper is, it turns the latter out, takes its place, setting the copper free, and forming, with the sulphur oxide, sulphuric acid. The liberated copper goes, and adheres to the copper plate, and, far from detracting from its efficacy, as the liberated hydrogen would have done, actually increases its efficiency, as it is deposited in a roughened form, which presents a large surface for the collection of the electricity. The interchange which takes place when the free hydrogen meets the sulphate of copper (outside the porous cells) is shown in the following equation:—

H2	+	CuSO4	=	H2SO4	+	Cu.
Free hydrogen	and	copper sulphate	produce	sulphuric acid	and	free copper.

Fig. 6. Daniell Cell.

§ 25. The original form given to this, the Daniell cell, is shown at Fig. 6, in which Z is the zinc rod standing in the porous pot P, in which is placed the dilute sulphuric acid. A containing vessel, V, of glazed earthenware, provided with a perforated shelf, S, on which are placed the crystals of sulphate of copper, serves to hold the copper sheet, C, and the solution of sulphate of copper. T and T' are the terminals from which the electricity is led where desired.

In another form, the copper sheet itself takes the form and replaces the containing vessel V; and since the copper is not corroded, but actually increases in thickness during action, this is a decided advantage. A modification, in which the porous cell is replaced by sand or by sawdust, is also constructed, and known as "Minotto's" cell: this, owing to the greater thickness of the porous layer, offers more resistance, and gives, consequently, less current. By taking advantage of the greater specific gravity (weight, bulk for bulk) of the solution of sulphate of copper over that of water or dilute sulphuric acid, it is possible to construct a battery which shall act in a manner precisely similar to a Daniell, without the employment of any porous partition whatsoever. Fig. 7 illustrates the construction of one of these, known as "Gravity Daniells."

Fig. 7. Gravity Cell.

In this we have a plate, disc, or spiral of copper, C, connected by an insulated copper wire to the terminal T'. Over this is placed a layer of crystals of copper sulphate; the jar is then filled nearly to the top with dilute sulphuric acid, or with a strong solution of sulphate of zinc (which is more lasting in its effects, but not so energetic as the dilute sulphuric acid), and on the surface of this, connected to the other terminal, T, is allowed to rest a thick disc of zinc, Z. Speaking of these cells, Professor Ayrton, in his invaluable "Practical Electricity," says:—"All gravity cells have the disadvantage that they cannot be moved about; otherwise the liquids mix, and the copper sulphate solution, coming into contact with the zinc plate, deposits copper on it. This impairs the action, by causing the zinc to act electrically, like a copper one. Indeed, without any shaking, the liquids mix by diffusion, even when a porous pot is employed; hence a Daniell's cell is found to keep in better order if it be always allowed to send a weak current when not in use, since the current uses up the copper sulphate solution, instead of allowing it to diffuse." The use of a solution of zinc sulphate to act on the zinc rod, or plate, is always to be preferred in the Daniell cell, when long duration is of more consequence than energetic action.

§ 26. There are many other bodies which can be used in batteries to absorb the hydrogen set free. Of several of these we need only take a passing notice, as the batteries furnished by their use are unfit for electric bell work. Of these we may mention nitric acid, which readily parts with a portion of the oxygen (§ 18) and reconverts the free hydrogen into water. This acid is used as the "depolarizer"[9] in the "Grove" and in the "Bunsen" cell. Another very energetic "depolariser" is chromic acid, either in solution, in dilute sulphuric acid, or in the form of potassic dichromate (bichromate of potash: bichrome). As one form of chromic cell has found favour with some bell-fitters, we shall study its peculiarities farther on.

Another class of bodies which readily part with their oxygen, and thus act as depolarisers, are the oxides of lead and manganese. This latter oxide forms the basis of one of the most useful cells for electric bell work, namely: the one known as the "Leclanché." As the battery has been, and will probably remain, long a favourite, the next paragraph will be devoted to its consideration.

§ 27. The Leclanché cell, in its original form, consists in a rod or block of gas carbon (retort scurf: graphite) standing in an upright porous pot. Around this, so as to reach nearly to the top of the porous cell, is tightly packed a mixture of little lumps of graphite and black oxide of manganese (manganic dioxide: black wad), the porous cell itself being placed in an outer containing vessel, which usually takes the form of a square glass bottle. A zinc rod stands in one corner of the bottle, and is prevented from coming into actual contact with the porous cell by having an indiarubber ring slipped over its upper and lower extremities. The glass containing vessel is then filled to about two-thirds of its height with a solution of ammonium chloride (sal ammoniac) in water, of the strength of about 2 oz. of the salt to each pint of water. This soon permeates the porous cell and reaches the mixture inside. The general appearance of the Leclanché cell is well shown at Fig. 8.