turn; and thus the jar forms a true condenser. When a communication is made by the hand or by both hands between its two faces, the two electricities accumulated there rush toward each other with great rapidity, and traversing the bodily organs produce in them a violent shock; or, which is the same thing, the body which is the medium of communication suffers a rapid decomposition of its natural electricities, each of which tends to that surface of the jar where the opposite electricity resides.

This explanation may be verified in every particular by ́experiments similar to those employed in the case of the condenser. Generally, the Leyden jar is simply a condenser, in which the insulating layer is curved, and which has for its coating or armour, as it is sometimes called, on the outside, the sheet of metal with which the jar is covered, and within, the conducting substance with which the jar is filled or covered.

89. When an electrified Leyden jar is suspended in the air, the absorbing action of that fluid can act only on the portion of electricity which is free upon either surface of the glass, and the reciprocal action of the two disguised electricities serves to protect them both. This is very evident from the long time which Leyden jars of thin glass take to discharge themselves completely, when they are insulated and when the direct communication of their two surfaces is interrupted by a layer of pure gum lac.

If we examine, at different times, the progress of this absorption, by touching the two surfaces with the trial plane, we shall find that there have been developed upon each quantities of free electricity, of a contrary nature, which finally become sensibly equal; after which they maintain themselves in this state of equality until both are completely exhausted. We are able, by means of the calculus, to account very exactly for this phenomenon, according to the laws of the absorption of electricity by the air. When, however, the equality of the charges is thus established upon the two surfaces, if we spread upon each a nonconducting powder, it would evidently adhere by the attraction of the free electricity; and if, moreover, the electricity were not strong enough to repel the particles, they would thus be preserved from the contact of the air; and thus, there being no waste, the jar will remain charged for an indefinite time. This

we in fact observe, when the two surfaces of a thin glass jar, after being charged, are covered with a mixture of sulphur and red lead, of which we have spoken above. If we suspend such a jar by a cord along a dry wall, it will preserve its electricity for months.

90. When we are employed in electrical experiments, we ought never to lose sight of the influence derived from the contact of the air. Overlooking this, we are apt to believe, for instance, that a Leyden jar, or other instrument of the kind, may be charged merely by receiving the electricity of the machine upon one of its faces, without communicating by the other with the ground; for, indeed, a jar thus insulated is gradually charged especially if it is electrified for a long time. But this is because the electricity of its other surface, repelled and rendered free by influence at a distance, is exposed to the absorbing action of the air which slowly diminishes it, and thus permits the accumulation of a certain quantity of electricity upon the surface communicating directly with the machine. To make this effect conspicuous, we have only to arm the outer surface with several points; the jar, although insulated in the air, is charged almost as strongly as if the surface armed with points had communicated directly with the ground.

Of the Electric Battery.

91. When we wish to accumulate a large quantity of electricity, we form several Leyden jars of a large size, coating the two surfaces with tin foil, and connecting the interior surfaces together, and the exterior together, so that when they are charged by communicating with the conductor of an electrical machine, they may all be discharged at the same time. This apparatus is called an electric battery; it is represented in figure 35. It is usually placed upon an insulating support, which communicates with a metallic conductor that may be removed and replaced at pleasure.

The greater the extent of armed surface a battery contains, the more electricity it accumulates, the action of the machine being the same; it requires also more time to charge it. GenE. & M.

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erally, when we make use of large batteries, it is useful to separate them into several parcels in order to be able to proportion the quantity of electricity to the effects to be produced. By this means we are able also to charge batteries more rapidly with the same machine.

92. Suppose any number of Leyden jars, or armed surfaces of glass, suspended under each other by metallic conductors, as represented in figure 36. We attach the first to a cord of silk S, and make the last communicate with the ground. We then convey upon the upper face A,, the electricity of the machine which we suppose vitreous; it is evident that all the lower plates will be charged at the same time with the first, by the successive repulsions of the electricity of one into the other. But both reasoning and experiment show, that in this way of charging by cascade, as it is called, the decomposition of the natural electricities is weakened very fast, as we recede from the prime conductor; so that if we take only a small number of plates, the last are scarcely charged at all. Moreover, if we make the first and last links of the chain communicate with each other by their opposite faces, we obtain the discharge of the quantities of electricity only which they have individually acquired; and those of the intermediate plates recombine of themselves without producing any effect; whereas we may avail ourselves of their power also, if, after having charged the system by cascade, we separate its successive parts in order to make the faces charged with the same electricity communicate with each other, and then discharge them simultaneously. The same method may be advantageously employed in charging large batteries. For this end, it is necessary to separate them into several parcels, and to place them upon insulating feet, as represented in figure 37. If we wish to charge them all or only a part of them, we at first establish a communication between the successive faces B1, A2, B2, A 31 by means of the metallic rods C1, C2, . . . . pass through rings provided for this purpose; and we make the last face B communicate with the ground. Afterward, when the charge is supposed to be sufficient, we destroy the communication of the face B2 with the ground. We may then safely remove, one after the other, the metal rods C1, C2 for when we remove C1, for instance, no discharge can take place, for

3,....

. which

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the electricity B, is entirely retained by A,, and the electricity A, almost entirely by B. Nevertheless, we shall thus receive a feeble spark arising from the excess of A, over B,. This being done, and the partial batteries being thus separated, we establish communications between their surfaces A1, A2, by throwing on the same metal rods C1, C.,... (if we lay them on, we should be exposed to a discharge ;) these rods meeting the conductors by which the parts of each battery are connected, naturally place them in communication. Each time the rod falls upon two consecutive parts, it excites a spark between them which comes from the inequality of the charges acquired during the first arrangement. When the batteries are all united, we can discharge them all at a single contact, by making the communication between the extreme faces A, and B2; or, if we please, we can first charge them completely by a renewed motion of the machine.

In these operations, it is important to have an electrometer, or, as it is sometimes called, a regulator to point out at each instant the state of the battery. For, at a certain point of intensity, the portion of electricity of the faces A may have a repulsive force sufficient to overcome the resistance of the air, and by rushing with an explosion toward a face B, the battery would be discharged, and some of the jars perhaps broken, because all the force of the shock tends then toward a single point of the coating. To avoid an accident of this kind, we screw upon the conductors communicating with the faces A, a small pendulum having a metallic rod TT, and a small rod of ivory carrying upon its extremity a small ball b of elder pith. The free fluid of the faces A, Fig. 38. exerting its repulsive force upon this pendulum, repels it from the stem; and its divergences are measured by a graduated arc traced upon the semicircle c c. It is evident that this instrument gives no absolute measure of the electricity accumulated; but affords at least a constant indication by which we can be guided, when we have determined by experiment, once for all, the degree of repulsion at which a spontaneous discharge is to be apprehended.

In discharging batteries, we make use of the exciter or discharger already described. We connect one extremity or knob with a face A, and the other with a face B, and the discharge

takes place through this conductor. When we have occasion to use large batteries, care should be taken how we expose ourselves, by becoming a part of the circuit; for a discharge through the body might be attended with serious consequences.

Fig. 39.

Of the Electric Pile and of the Phenomena presented by Crystals capable of being electrified by Heat.

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93. While on the subject of charging by cascade, I shall present some results which will be found useful hereafter when we come to treat of galvanism and magnetism. They will also afford some new examples of the action of disguised electricity. Imagine a series of glass plates, having the two surfaces coated with metal, and arranged parallel to each other in such a way that the face B, of the first shall communicate by a wire with the face A, of the second; the face B2 of the second with the face A ̧ of the third; and so on to the last, the lower face B of this last communicating with the ground. Let us suppose that, the whole apparatus being insulated, we make the first face A, communicate with the prime conductor of a powerful machine, and that after having thus electrified it by cascade for some time, we interrupt the communication with the conductor and with the ground by means of non-conducting rods. It is proposed to find what will be, after a certain interval, the electrical state of the different parts of the apparatus.

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To determine this, it is necessary to consider that at the moment when the communication is broken, the first face A contains a certain electrical charge, in part free, and in part disguised by the electricity of a contrary nature which it has itself attracted and fixed upon the second face B1; it is the same with the faces A, and B,, with A, and B,, and so on through all the others. Of all these quantities there is only the charge A, which is foreign to the apparatus; all the others being derived from the simple decomposition of the natural electricities. The absolute intensity of decomposition varies from one plate to another; but all which is excited upon each is not sensible; there is nothing sensible except the portions of free electricity, which are all of the same nature with that belonging to A ̧.