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§ 1. Electricity.—The primary cause of all the effects which we are about to consider resides in a force known as electricity, from the Greek name of amber (electron), this being the body in which the manifestations were first observed. The ancients were acquainted with a few detached facts, such as the attractive power acquired by amber after friction; the benumbing shocks given by the torpedo; the aurora borealis; the lightning flash; and the sparks or streams of light which, under certain conditions, are seen to issue from the human body. Thales, a Grecian philosopher, who flourished about 600 years B.C., observed the former of these facts, but nearly twenty centuries elapsed before it was suspected that any connection existed between these phenomena.

§ 2. According to the present state of our knowledge, it would appear that electricity is a mode of motion in the constituent particles (or atoms) of bodies very similar to, if not identical with, heat and light. These, like sound, are known to be dependent on undulatory motion; but, whilst sound is elicited by the vibration of a body as a whole, electricity appears to depend, in its manifestations, upon some motion (whether rotary, oscillatory, or undulatory, it is not known) of the atoms themselves.

However this be, it is certain that whatever tends to set up molecular motion, tends also to call forth a display of electricity. Hence we have several practical means at our disposal for evoking electrical effects. These may be conveniently divided into three classes, viz.:—1st, mechanical; 2nd, chemical; 3rd, changes of temperature. Among the mechanical may be ranged friction, percussion, vibration, trituration, cleavage, etc. Among the chemical we note the action of acids and alkalies upon metals. Every chemical action is accompanied by electrical effects; but not all such actions are convenient sources of electricity. Changes of temperature, whether sudden or gradual, call forth electricity, but the displays are generally more striking in the former than in the latter case, owing to the accumulated effect being presented in a shorter time.

§ 3. We may now proceed to study a few of these methods of evoking electricity, so as to familiarise ourselves with the leading properties.

If we rub any resinous substance (such as amber, copal, resin, sealing-wax, ebonite, etc.) with a piece of warm, dry flannel, we shall find that it acquires the power of attracting light bodies, such as small pieces of paper, straw, pith, etc. After remaining in contact with the rubbed (or electrified) substance for a short time, the paper, etc., will fly off as if repelled; and this apparent repulsion will be more evident and more quickly produced if the experiment be performed over a metal tray. If a small pith-ball, the size of a pea, be suspended from the ceiling by a piece of fine cotton, previously damped and then approached by an ebonite comb which has been briskly rubbed, it will be vigorously attracted, and never repelled; but if for the cotton there be substituted a thread or fibre of very fine dry silk, the pith-ball will be first attracted and then repelled. This is owing to the fact that the damp cotton allows the electricity to escape along it: id est, damp cotton is a CONDUCTOR of electricity, while silk does not permit its dissipation; or, in other words, silk is a NON-CONDUCTOR. All bodies with which we are acquainted are found, on trial, to fall under one or other of the two heads—viz., conductors and non-conductors. Nature knows no hard lines, so that we find that even the worst conductors will permit the escape of some electricity, while the very best conductors oppose a measurable resistance to its passage. Between the limits of good conductors, on the one hand, and non-conductors (or insulators) on the other, we have bodies possessing varying degrees of conductivity.

§ 4. As a knowledge of which bodies are, and which are not, conductors of electricity is absolutely essential to every one aspiring to apply electricity to any practical purpose, the following table is subjoined, giving the names of the commoner bodies, beginning with those which most readily transmit electricity, or are good conductors, and ending with those which oppose the highest resistance to its passage, or are insulators, or non-conductors:—

§ 5. TABLE OF CONDUCTORS AND INSULATORS.

Quality.	Name of Substance.	Relative Resistance.
Good Conductors	Silver, annealed	1.
Copper, annealed	1.063
Silver, hard drawn	1.086
Copper, hard drawn	1.086
Gold, annealed	1.369
Gold, hard drawn	1.393
Aluminium, annealed	1.935
Zinc, pressed	3.741
Brass (variable)	5.000
Platinum, annealed	6.022
Iron	6.450
Steel, soft	6.500
Gold and silver alloy, 2 to 1	7.228
Nickel, annealed	8.285
Tin, pressed	8.784
Lead, pressed	13.050
German silver (variable)	13.920
Platinum-silver alloy, 1 to 2	16.210
Steel, hard	25.000
Antimony, pressed	23.600
Mercury	62.730
Bismuth	87.230
Graphite	145.000
Nitric Acid	976000.000
Imperfect Conductors	Hydrochloric acid	[1]
Sulphuriacid	1032020.000
Solutions of metallic salts	varies with strength
Metallic sulphides	[1]
Distilled water	[1] 6754208.000
Inferior Conductors.	Metallic salts, solid	[1]
Linen	}	and other forms of cellulose	[1]
Cotton
Hemp
Paper
Alcohol	[1]
Ether	[1]
Dry Wood	[1]
Dry Ice	[1]
Metallic Oxides	[1]
Non-conductors, or Insulators.	Ice, at 25 c.	[1]
Fats and oils	[1]
Caoutchouc	1000000000000.
Guttapercha	1000000000000.
Dry air, gases, and vapours	[1]
Wool	[1]
Ebonite	1300000000000.
Diamond	[1]
Silk	[1]
Glass	[1]
Wax	[1]
Sulphur	[1]
Resin	[1]
Amber	[1]
Shellac	[1]
	Paraffin	1500000000000.

[1] These have not been accurately measured.

The figures given as indicating the relative resistance of the above bodies to the passage of electricity must be taken as approximate only, since the conductivity of all these bodies varies very largely with their purity, and with the temperature. Metals become worse conductors when heated; liquids and non-metals, on the contrary, become better conductors.

It must be borne in mind that dry air is one of the best insulators, or worst conductors, with which we are acquainted; while damp air, on the contrary, owing to the facility with which it deposits water on the surface of bodies, is highly conducive to the escape of electricity.

§ 6. If the experiment described at § 3 be repeated, substituting a glass rod for the ebonite comb, it will be found that the pith-ball will be first attracted and then repelled, as in the case with the ebonite; and if of two similar pith-balls, each suspended by a fibre of silk, one be treated with the excited ebonite and the other with the glass rod, until repulsion occurs, and then approached to each other, the two balls will be found to attract each other. This proves that the electrical condition of the excited ebonite and of the excited glass must be different; for had it been the same, the two balls would have repelled one another. Farther, it will be found that the rubber with which the ebonite or the glass rod have been excited has also acquired electrical properties, attracting the pith-ball, previously repelled by the rod. From this we may gather that when one body acting on another, either mechanically or chemically, sets up an electrical condition in one of the two bodies, a similar electrical condition, but in the opposite sense, is produced in the other: in point of fact, that it is impossible to excite any one body without exciting a corresponding but opposite state in the other. (We may take, as a rough mechanical illustration of this, the effect which is produced on the pile of two pieces of plush or fur, on being drawn across one another in opposite directions. On examination we shall find that both the piles have been laid down, the upper in the one direction, the lower in the other.) For a long time these two electrical states were held to depend upon two distinct electricities, which were called respectively vitreous and resinous, to indicate the nature of the bodies from which they were derived. Later on (when it was found that the theory of a single electricity could be made to account for all the phenomena, provided it was granted that some electrified bodies acquired more, while others acquired less than their natural share of electricity), the two states were known as positive and negative; and these names are still retained, although it is pretty generally conceded that electricity is not an entity in itself, but simply a mode of motion.

§ 7. It is usual, in treatises on electricity, to give a long list of the substances which acquire a positive or a negative condition when rubbed against one another. Such a table is of very little use, since the slightest modification in physical condition will influence very considerably the result. For example: if two similar sheets of glass be rubbed over one another, no change in electrical condition is produced; but if one be roughed while the other is left polished, this latter becomes positively, while the former becomes negatively, electrified. So, also, if one sheet of glass be warmed, while the other be left cold, the colder becomes positively, and the latter negatively, excited. As a general law, that body, the particles of which are more easily displaced, becomes negatively electrified.

§ 8. As, however, the electricity set up by friction has not hitherto found any practical application in electric bell-ringing or signalling, we need not to go more deeply into this portion of the subject, but pass at once to the electricity elicited by the action of acids, or their salts, on metals.

Here, as might be expected from the law enunciated above, the metal more acted on by the acid becomes negatively electrified, while the one less acted on becomes positive.[2] The following table, copied from Ganot, gives an idea of the electrical condition which the commoner metals and graphite assume when two of them are immersed at the same time in dilute acid:—