lated bodies A, all of the same nature, and in their natural state, which are rubbed successively against the same insulated body B. Now when the repetition of this friction has developed in B, the maximum of electricity which can be maintained upon this body in contact with A, notwithstanding the repulsive force which this electricity possesses, it is manifest that new friction with other bodies A, cannot produce in B any new development. of electricity. For if a new quantity - e' should be developed and should unite itself with e, the whole repulsive force e e' would overcome the resistance which opposes its diffusion over the surface in contact; and thus the new quantities of decomposed electricity would be immediately recompounded. Such must also be the result of the continued friction of the plate of the electrical machine against the cushions when they are insulated. The parts of the plate which first present themselves immediately develope in the cushions all the electricity which can be maintained upon them under the influence of the friction; after which the contact of the succeeding parts produces none, and the development of the electricity ceases, so that the plate no longer offers any thing to be neutralized to the prime conductor, whatever number of turns it may make. On the contrary, if the cushions communicate with the ground, and are thus constantly maintained in their natural state, the parts of the plate as they return successively, after being discharged by the prime conductor, are together with the cushions in the same state as at the first contact. They may therefore produce again in the cushions the decomposition of the natural electricities, become charged with a portion of the vitreous necessary to an equilibrium in this case, and come again to be neutralized by passing before the prime conductor, whence this electricity spreads itself over the secondary conductors, upon the surface of which it distributes itself according to the laws of electrical equilibrium; and this continual development of electricity ceases only when the whole quantity thus spread through the entire system of conductors, has acquired such a repulsive force that its action upon the

prime conductor nearest the plate, instead of being armed with points, had only an angular form, so that the escape of the electricity might easily take place.

Fig. 21.

part of the prime conductor nearest the plate, shall equal the opposing action exerted by the electricity, also vitreous, adhering to the parts of the plate presented to the conductor. It is then useless to continue the motion of the machine; the charge of the prime conductor does not increase; or at most, it only acquires what is necessary to replace the waste occasioned by the air coming in contact with all the electrified surfaces of the plate and conductor.

This minute analysis of the phenomena of the electrical machine will suggest to us several important particulars by which its construction may be improved.

62. (1.) It is necessary that the parts of the glass which have been successively rubbed, should come before the conductor with the least possible loss of the electricity they have acquired. For this purpose, we attach to the rubber pieces of oiled silk or gummed taffeta, extending over the surface of the glass in the direction of the motion. After the glass is electrified, these strips adhere to its surface, and preserve it from the contact of the air till it has come near to the prime conductor.

(2.) It is necessary that the prime conductor should have as many branches as there are rubbers. We usually employ two rubbers F and F', each of which comes in contact with both surfaces of the plate. They are placed at the two opposite extremities of the same diameter of the plate; and in order to establish with certainty their communication with the ground, the back part of each rubber consists of a piece of metal communicating with the two metallic branches AM, AM', which depart from the axis of rotation AA' also metallic. We have then only to connect this with the ground; for this purpose we attach to it a chain extending to the floor of the room, or, which is much better, communicating by means of a system of conductors with a water pipe or well. The prime conductor consists also of two branches CB, CB', the parts of which nearest the plate are armed with points for the purpose of discharging more easily the resinous electricity developed there by the vitreous influence of the parts of the plate successively presented to them. But the opposite extremities of these branches we never arm with points which would rapidly dissipate into the air the electricity acquir ed by the conductor; on the contrary, they are made to termi

nate in a large ball. Still a conductor thus terminated would be saturated with a moderate quantity of electricity. On this account it is made to communicate with a system of insulated conductors, formed of long and narrow cylinders suspended parallel to each other. Experiment and theory concur to show, that where the lengths and diameters of these cylinders are in proper proportion, this arrangement is best adapted to obtain large charges with but feeble intensities. It has this advantage

also, that when we come to turn the plate or cylinder, we can cut off the communication between the prime and secondary conductors; for by this means we prevent the dissipation of the accumulated electricity which would rapidly escape by the points of the prime conductor, when the electricity of the plate, by not being renewed, should cease to repel it.

It is evident that these changes in the communication ought not to be made by the direct contact of the hands of the experimenter, but by means of metallic rods attached to insulating handles. When only a momentary communication is required, we usually give to these rods the form of two circular arcs AC, A' C, turning on a hinge about the centre C, and each provided with an insulating handle M, which ordinarily is a rod of glass covered with gum lac. We take one of these rods in the left hand, the other in the right; then opening or closing the angle which they form, we can augment or diminish at pleasure the distance AA' of the two extremities of the arc, and adapt it to the distance between the two conductors which we wish to connect. This instrument is called an exciter, because it in fact serves to excite sparks between one conductor and another. The instrument represented in figure 24 answers the same purpose, although it is generally used to discharge jars or batteries, and is hence called a discharger. We also employ, as means of communication, metallic chains and cords which are suffered to hang from one conductor to another, and which are easily removed with tubes of glass when we wish to cut off the communication.

63. After determining the best forms for all the parts of an electrical machine, it only remains to say a word respecting insulation. It is plain that the insulation of the prime and secondary conductors ought to be as perfect as possible, that they may preserve for a long time the electricity which has been E. & M.

10

Fig. 22.

Fig. 23.

communicated to them. For this purpose, the supports should be as long and thin as consists with convenience and stability. Those of the prime conductor are usually glass pillars. They should be varnished with gum lac, because this gum insulates much better than glass, and is less likely to contract moisture. The secondary conducters may be suspended from the ceiling by silk cords; and it would be well, in this case, if the upper part of the cords were terminated by a cylinder of gum lac. As to other particulars, we proceed according to the principles laid down in articles 16 24.

64. We have thus far supposed the rubbers to communicate with the ground, and the conductors to be insulated. In this case the electricity acquired by the conductors is vitreous. But we may also give them the resinous electricity. For this purpose, we make the branches CB, CB', of the prime conductor moveable about the axis CC', and also the two branches AM, AM', which connect the rubbers with the ground. If we would obtain the resinous electricity, we turn these branches, as represented in figure 25, so that those of the prime conductor, which are insulated, shall touch the pieces of metal on the back of the rubbers respectively, and those which before communicated from the rubbers to the ground are to be placed opposite to the rubbed surfaces of the plate. Then the vitreous electricity acquired by the plate is neutralized in a degree by the resinous electricity thus developed by influence in the branches AM, AM'; and, on the contrary, the prime conductor retains all the resinous electricity which is developed upon the rubbers. With this disposition of the instrument, it is necessary that the points with which the branches of the prime conductor are armed, should be disposed in such a manner, as to be opposite to, or in contact with, the rubbers, in order that their resinous electricity may pass into the system of conductors, either immediately and by contact, or by influence. Moreover, the supports which sustain the cushions and which are usually attached to the frame. work of the machine, ought, in this case, to be of an insulating nature, and so arranged as to produce the most perfect insulation. It is also important to be able, as we have supposed, to bring before the glass plate the two metallic branches AM, AM', which communicate with the ground, in order to neutralize all the vit

reous electricity with which the surface is covered when it comes from the rubbers; for, if it preserved this electricity, it would develope none anew when it passed a second time between the cushions, and the charge of resinous electricity which the conductor might acquire, would be much less.

Of Electroscopes.†

65. Electroscopes are instruments destined, as their name imports, to discover the smallest quantities of electricity. We have already spoken of that of Coulomb, which is a true elec- 9: trical balance suspended by a thread of silk as it comes from the silk worm. Other electroscopes are also founded on the general principle of the repulsion which takes place between bodies charged with similar electricities; and their greater or less sensibility depends on the lightness and facility of motion of the substances employed to manifest this repulsion. These are usually two long light pieces of straw, or two slips of gold Fig. 26. leaf L, L', suspended parallel and very near each other, by means of two very fine pieces of wire that hook into the rings a, a', formed in a common stem or rod, also metallic, which is terminated by a knob. By means of this continued communication, all the electricity given to the rod T is spread over the wires, and thence over the straws or leaves, which immediately manifest it by diverging from each other. But since the portion communicated is in fact all which is indicated, it must be evident that the apparatus will be the more sensible, according as these slips are lighter, more free in their motion, and according as the rod T, which communicates the electricity, retains a less portion of it upon its own surface. For this reason, it is necessary that the stem should be thin and the knob small, though of a size much greater than the stem. To prevent any motion from the air, and to screen it from accidental injury, the whole apparatus is enclosed in a square glass case, the neck of which Fig. 27. is covered with gum lac that the insulation may be more perfect.

Usually called electrometers in English treatises on Electricity.