remarkable. We have seen that the pointed form determines, in the part of the conductor that possesses it, a charge or electric reaction so much more considerable than in the rest of its surface that the electricity most frequently escapes from it to go either into the air, or upon the nearest conductors. So, when an electrised body is presented to a conductor, that is terminated in a point and insulated, the electricity of the contrary nature, that is developed by induction in the conductor, by accumulating at its point, escapes from it to neutralise through the air that of the electrised body: thus it is that the conductor of an electrical machine may be discharged, or may be prevented from charging, by presenting to it at a distance of eight or ten inches, or even further, a metal point, held in the hand. In the working of the electrical machine a phenomenon of the same kind takes place. Each part of the glass plate that has been electrised by friction passes successively before the points of the insulated conductor of the machine, whose natural electricity it decomposes by induction, attracting the negative and repelling the positive. The negative, being accumulated at the points, escapes from them to neutralise the positive of the plate, which, on passing anew between the cushions, regains, by the friction it there undergoes, the positive electricity it has lost. With regard to the positive electricity of the conductor: being deprived of the negative that has escaped from it by the points, it is no longer able to produce the neutral fluid, and remains consequently in excess; on this account it is, that after a certain number of turns the insulated conductor of the machine is found to be charged with positive electricity. It is not, therefore, as has often been erroneously said, the positive electricity of the glass plate that has passed into the conductor: it is the negative of the conductor that has passed out by the points, and has left there the positive, with which it formed the neutral fluid, and from which it has been separated by the influence of the plate. This inductive action continues to accumulate positive electricity in the conductor until each point of the surface of the latter has a reaction equal to what each point of the surface of the glass acquires by its friction

against the cushions. Indeed, when this limit is obtained, there is no reason why the electricity of the conductor should not act just as much upon the plate, as the electricity of the plate upon the conductor. There is, therefore, equilibrium ; and the conductor takes no further charge: we can therefore understand that the charge of the conductor must be higher according as the friction determined upon the plate is more energetic; an energy which itself depends on the quality of the glass, on that of the rubbers, on the manner in which they are more or less properly adjusted, on the degree of dryness of the air, and on the greater or less care that has been taken to dry and clean the surface of the plate itself.

Action at a Distance by Induction upon Electroscopes.

It is also to the developement of electricity by induction that we must connect the fact of its not being necessary to touch an electroscope with an electrised body in order to act upon it, but that it is sufficient to approach it with this body at some distance. Indeed this body, by its influence, decomposes the natural electricity of the metal parts of the electroscope situated exteriorly, attracts near to it the electricity of the contrary name to its own, and repels that of the same name into the gold leaves, the blades of straw, or other light substances that are in communication with this metal part. The electroscope is thus found charged, while under the influence of the electrised body, with the same electricity as that possessed by this body, but, in order to preserve it, it must then be touched with the body itself. However, if, whilst it is subjected to the inductive action, we touch it with the finger on any part of its exterior metal part, it is then found charged with an electricity contrary to that of the electrised body, provided we have taken care to remove the finger before withdrawing this body. The fact is, the electricity of the same name, in obeying the repulsive action, instead of having been driven into the gold leaves, has escaped away into the ground by the intervention of the body and finger of the observer. There therefore remained in the

instrument, when it was withdrawn from the inductive action, an excess of the contrary electricity; which caused its gold leaves to diverge. This is a more expeditious and more convenient mode of charging the electroscope: but we must not forget, when we adopt it, that the electricity which it indicates is of the contrary nature to that of the body with which we acted upon it.

Electrophorus.

An instrument, founded upon the principle of the developement of electricity by induction, and which may in many cases advantageously supply the place of the electrical machine, is that contrived by Volta, the Electrophorus. It is composed of a cake of resin poured into a circular mould of wood or metal of any diameter. A disc of metal, or of wood covered with tin, and of a less diameter than that of the cake, is furnished with an insulating handle fixed at its centre perpendicularly to its surface. This disc is bounded by a rounded edge, so as to avoid sharp corners, by which the electricity would run the risk of escaping (Fig. 43.). The cake of resin is electrised by beating its surface with a cat's skin; the metal disc is then placed upon it, holding it by the metal handle; we touch it with the finger, and, when it is raised, we find it charged with positive electricity. We may repeat the experiment a very great number of times, and even at several days apart, without its being necessary to electrise the cake anew. The latter preserves the resinous electricity that has been developed upon its surface for a very long time, on account of its insulating property, and its small tendency to attract moisture from the air. We must only take care to let the metal disc remain upon it, the presence of which prevents the loss of electricity that would result from the contact of the air. It is not necessary to add that it is the negative electricity of the cake which, by decomposing the natural electricity of the metal disc, drives the negative into

Fig. 43.

the ground by means of the finger, and attracts the positive, which is found in the disc, as soon as it is raised. If we raise it without having previously touched it with the finger, we then find it charged, not with positive electricity, but with a certain quantity of negative electricity, which it has taken from the cake by simple communication. This quantity is always very feeble on account of the difficulty the electricity experiences in quitting the resin.

The positive electricity with which the disc is charged is sufficiently energetic to give strong sparks; and hence it is used for inflaming gases and for a great number of experiments. There is also an apparatus called the electric lamp, wherein a jet of hydrogen is inflamed by the spark given by an electrophorus, the metal disc of which, by means of a silk cord fixed to a stop-cock whereby the gas escapes, is detached from the cake of resin by the same movement that opens the stop-cock. An insulated conductor, that just touches the disc when it is raised by the movement of the stop-cock, is terminated,

O

Fig. 44.

towards the jet of hydrogen, by a fine point placed opposite, and at a very small distance from a similar point. communicating with the ground. It is between these two points that the electric spark passes, which thus meets and inflames the jet of hydrogen gas. This gas is itself produced afresh in the apparatus, in proportion as it passes away, by means of a zinc rod, which is acted upon by water acidulated with sulphuric acid (Fig. 44.); care, however, must be taken, in order that the instrument may work well, to electrise the cake of resin from time to time with a cat's skin.

Explanation of the Attraction of light Bodies by electrised Bodies.

The primitive phenomenon of electricity, that of the attraction of light bodies by an electrised body, is less simple than is commonly supposed. It is a true phenomenon of induction. There is no attraction, neither is there any repulsion, between an electrised body and a body that is not so; there is none except between bodies both electrised. Thus when a stick of rubbed wax is presented to light bodies, such as bits of paper or pith balls, the stick of wax decomposes their natural electricity, repels into the ground their negative electricity; they, still retaining an excess of positive electricity, obey the attraction which a negatively electrised body must exercise on a body positively electrised.

In support of this manner of interpreting the phenomenon, we may quote the following facts:

The first is that, if the light bodies are of an insulating material, such as resin or glass, they are not attracted by an electrised body, because their natural electricity cannot be decomposed so easily as when they are conductors. The second fact is, that if the shreds of paper or the other light bodies rest upon an insulating surface, such as a plate of glass or resin, they are no longer so easily attracted, because the one of their two electricities that is repelled, cannot quit them to pass into the ground: then the other electricity cannot but with difficulty overcome by its attractive power the repulsive force of the former. That there may be no attraction at all, it is necessary that the light bodies be thin and small; pith balls, if their diameter exceeds the tenth of an inch, are attracted when the electrised body is brought very near to them, even when they are placed upon an insulating surface because the contrary electricity to that possessed by the body is, in a portion of their surface, that is sensibly nearer than is that portion in which the electricity of the same name is accumulated.