or two gold leaves beaten very thin, as in Bennet's electrometer (Fig. 14.); in this latter two small metal stems,

Fig. 12.

Fig. 13.

Fig. 14.

each terminated by a knob, are placed vertically on the foot of the instrument, on each side of the gold leaves, so that, when the latter diverge too strongly, they come in contact with the stems, which are in communication with the earth, and are thus discharged. We thus avoid the tearings resulting from the adhesion to the sides of the bell-glass, to which the gold leaves would be subject by the effect of too strong a charge of electricity. Finally; in all electroscopes a circular division enables us to measure the angular separation of the two light bodies, the pith balls, the blades of straw, or the gold leaves. The metal stem, in the gold leaf electrometer, carries, at its lower extremity, which penetrates into the upper part of the bell-glass, pincers, by means of which the gold leaves are fixed; and its upper extremity, which is outside the bell, is terminated by a disc or a knob. In order that the electricity which is given to the knob may not be lost, and may pass onward and affect the gold leaves, care is taken to place the metal rod itself in a glass tube, which covers it entirely, except at its two extremities: this tube penetrates into the bell through a metal ferrule, to which it is cemented by means of an insulating material, such as wax or gum-lac. The ferrule is also fixed by means of wax to the tubular hole of the bell (Fig. 15.).

Fig. 15.

It is very important that the air be very dry, both in the

interior of the bell and on its exterior surface, so that the electricity may not be dissipated by contact with moist air. With this view, the electroscope itself is frequently placed all entire in a glass cage, filled with chloride of calcium or quick lime, which well dries the air; care being taken to let no part be outside the cage, except the knob or metal disc, by which the electricity is made to reach the gold leaves.

With sensible electroscopes, such, in particular, as the gold leaf, it is not always necessary to touch with the electrised substance the exterior part of the metal rod which carries the light bodies, in order to pass into the latter the electricity that we wish to collect. It is sufficient to bring the substance near the rod; and, by an effect which we shall soon study, the gold leaves, while under this influence, are found to possess an electricity similar to that possessed by the substance itself. In like manner, if the electroscope is electrised, it is sufficient to bring near to it, without contact being necessary, a body whose electricity is known, in order to judge, from the greater or more feeble divergence of the two light bodies, what the nature is of the electricity with which they are charged.

Although the degree of divergence of gold leaves, and in general of all other light bodies, is in relation with the intensity of the electricity that animates them, this intensity is far from being exactly proportional to the number of the degrees of separation. Thus, to speak truly, the instruments that we have been describing deserve rather the name of electroscopes than that of electrometers, which latter name must be reserved to apparatus, such as Coulomb's balance, of which we shall speak in the following Part, and which give indications proportional to the intensity of the electricity with which they are charged.

CHAP. IV.

OF THE VOLTAIC PILE AND VOLTAMETERS.

Volta's Column Pile.

THE voltaic pile, like the electrical machine, is an apparatus in constant requisition in the study of electricity; but it is based upon another mode of developing the electric fluids. Friction is, indeed, not the only mode of producing this development: there exist others; in particular, elevation of temperature, and, also, the chemical action of one body upon another. The simple contact of two heterogeneous substances, such as that of two different metals, is also, according to some philosophers, a source of electricity. It is not the place here to study the different modes of producing electricity; this subject will be treated of in the Fifth Part.

We must here confine ourselves to saying that the Voltaic Pile is an apparatus in which electricity is developed, according to some by the contact of two metals of a different nature, and according to others by the chemical action of the liquids, with which it is charged, upon one of the two metals that enter into its formation. It presents, in fact, these two circumstances united: we shall see, in the sequel, what part must be attributed to each. This is of little consequence to us at present, since we are only concerned in the description of the apparatus. We may further add that the pile devised by Volta owed its origin to the interpretation which this celebrated philosopher gave to a remarkable experiment made by Galvani, namely, that a frog undergoes a violent commotion when one of its nerves, being exposed, is touched with one metal, and its muscles with another metal, the

Fig. 16.

two metals being themselves in close contact, in one or more points of their surface (Fig. 16.). This effect, which is due to a liberation of electricity, has caused the electricity thus liberated to be called galvanic, and the part of physical science concerned in it to be called galvanism; but the name of voltaic must remain to the pile, since it truly originates with Volta.

Fig. 17.

The first form that was given by Volta to the pile is that of a vertical column, formed of discs of copper and zinc, from 14 to 24 inches in diameter, arranged as follows (Fig. 17.). The base of the column is a copper disc, upon which is placed a zinc disc; the combination of these two superposed discs forms a pair; over this first pair a second similar pair is placed, care being taken that the copper is always below the zinc: the second pair is separated from the first by a circular piece of cloth or pasteboard, well moistened with water, or, which is better, with salt water or acid water. Upon the second pair is placed a third, arranged in the same manner, and separated also by a moistened circular piece, similar to that which preceded. In this manner a greater or less number of pairs are superposed one over the other, care being taken to retain them in their position by means of vertical rods of glass; if the precaution has been taken to insulate the pile by resting its base upon a plate of glass, it is found to be charged with negative electricity at its lower extremity, where it is terminated by the copper disc, and with positive electricity at its upper extremity, where it terminates by the disc of zinc. These extremities are termed poles; the former the negative, and the latter the positive pole of the pile. Had the two metals been placed in another order, namely, had we commenced with the zinc, and placed upon it the copper disc, then the moistened cloth, and then again zinc, copper, and moist cloth, and so on, the positive pole would have been below and the negative above. Two wires lead one from the extreme copper and the other from the extreme zinc, each communicating the electricity of the pole

whence it originates; and when they are brought together, a spark passes between them, resulting from the neutralization of the two contrary electricities. If these wires are held one in each hand, when the number of pairs in the pile is sufficiently great, a series of shocks are felt, the sensation of which is sometimes very painful. When, instead of the human body, a very fine wire of iron, platinum, or any other metal, an inch or two in length, is employed to connect the two conductors, the neutralization of the two electricities is brought about through this wire, which rises in temperature and becomes incandescent. The length and diameter of wire that can be heated are greater in proportion as the pile is more powerful. The most remarkable circumstance is, that the incandescent condition of the wire is permanent, because the neutralization of the two electricities is continued, the pile liberating them at each of its poles, in proportion and as rapidly as they are neutralized.

The two wires, that come from the poles, may also be plunged into water, which has been rendered saline or acidulated to make it a better conductor. In this case the submerged part of the two wires must be either of gold or of platinum. It will immediately be perceived that, through the continued neutralization of the two electricities which takes place by means of the water, the latter is decomposed, and its two constituent gases are liberated, the one, oxygen, around the wire that communicates with the positive pole; the other, hydrogen, around the wire, that communicates with the negative pole; the two gases are constantly in the same proportions that constitute water, namely, one volume of oxygen to two of hydrogen. In order to collect them, the precaution is taken of placing above each of the wires, which penetrate interiorly into the vessel containing the water, and to which they are well cemented, a tube, closed above and containing the same liquid as the vessel itself; and which is driven out by the gas in proportion as it ascends (Fig. 18.).

Fig. 18.

The phenomena, that we have been describing, are merely