When the prime conductor is placed in its proper position it receives from the glass, without contact with it, a charge in the form of a stream of fire, which, whatever may be the size of its surface, rises to nearly the same intensity as that of the original source; and which, being thus accumulated in quantity, will wholly pass off at once to any uninsulated conductor, or will instantaneously divide itself with an insulated one, by means of a dense spark. Similar phenomena may be obtained with the conductor attached to the rubber; but to obtain the highest effect from either, it is necessary to make a good conducting communication from the other to the ground. The reason of this is, that when the two electricities are in presence of each other, they counteract and limit each other's intensity: by connecting either conductor with the earth, its charge is spread over an indefinitely large surface, and virtually annihilated. That the two conductors are in opposite electrical states, is easily proved by suspending from each some light substances, which will strongly attract each other when charged and that the two charges are exactly equal, is shown by making a good metallic communication between the two, when all signs of excitement will cease in both.

§ 211. And now, it may be asked, Where does the charge reside in a good insulated conductor? Does it diffuse itself, like heat, throughout the whole of its substance, or is it confined merely to its surface? That it is merely superficial is susceptible

circular glass plate, mounted upon a brass axis, and turning in a stout wooden frame by means of the handle F; as it revolves it passes between two pairs of cushions B B, which are pressed lightly against it by means of screws; c is the brass prime conductor, supported upon the stout glass arm D; it is armed at its two extremities, which are opposed to the plate, with two rows of points, which meet the ends of the two silk flaps E E, which are attached to the cushions B B.

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of proof in several ways. If we take a solid globe of metal, and electrify it to a certain amount, and then bring it in contact for a moment with a globe of similar dimensions, made of the thinnest shell of the same metal, we shall find that the charge will distribute itself equally between the two. If we take a hollow sphere, with an aperture at its up per part, and, having given it a charge of electricity, touch its interior with a small insulated ball, upon bringing this proof-ball in contact with an electroscope, we shall find that it will afford no signs of having received a charge; but, on the contrary, if we touch the exterior of the sphere, it will carry away a part of the charge with it, and affect the in

strument.

How much the intensity of the electric charge depends upon the surface, may also be very elegantly shown by means of a metallic riband, coiled up by a spring upon the top of a gold-leaf electrometer. When this apparatus is electrified, the leaves, of course, repel one another in proportion to the charge which they receive. If the riband be now drawn out by a silk thread or other insulating handle, the leaves will approach each other in proportion to the enlargement of the surface over which the electricity becomes diffused; and, as it again coils itself up, they will expand to their original amount. Here it is seen that, the quantity of the charge remaining the same, its intensity decreases with the increase of the surface over which it is suffered to diffuse itself, and that the quantity of electricity which a given portion of matter may receive depends upon the dimensions of its external surface.

212. The next question which presents itself is, How does the electricity arrange itself around surfaces of different forms? in a layer of equal intensity in every part, or otherwise? This question is readily submitted to experiment by means of a

small insulated disc, which, being applied to any part of the surface of an electrified conductor, becomes virtually a part of that conductor; and, upon being removed, carries with it a portion of the charge, having the same intensity as that of the point to which it was applied, and which may be measured by the torsion electrometer.

In this way it has been proved that, in the case of an electrified sphere, the intensity is the same at every part of the surface; but this is the only form of surface upon which this equal distribution takes place. If two similar spheres be placed in contact with one another, it will be found that there are two points of greatest and equal intensity on their opposite sides, in a line with their points of contact where the force will be null. So in a cylinder or bar of metal, the electrical intensity will be much greater at the two ends than in the middle, and this inequality of distribution will increase very rapidly in proportion to the diminution of the diameter of a cylinder of given length.

If two balls of equal diameters be placed together, the maximum intensity of the extreme point of the smaller sphere will be higher than that of the corresponding point of the larger; and by adding a series of balls in contact with each other, all gradually decreasing in size, the intensity will increase upon the smaller as the diameter decreases. We can conceive a succession of such balls gradually diminishing till the series ends in a mere point, at which the electric tension will be at its maximum. In consequence of this law of distribution, a powerful dispersion of electricity takes place from all bodies of a pointed form; the intensity upon them increasing to such an extent that the surrounding insulating medium of air gives way before it, and no longer suffices to constrain it.

213. Our attention has hitherto been directed to the electricity which is developed upon the sur

faces of two dissimilar substances by mutual friction, or which has been transferred to other bodies from such excited substances by contact and direct communication. We have now to examine a remarkable influence which an electrified body exerts upon other bodies at such distances from it as prevent the direct transfer of any portion of the charge. The neutral state of an insulated conductor in its immediate vicinity will be destroyed. If it be in the form of a cylinder, furnished with electroscopes at each extremity and also at its centre, when one end is placed near the charged substance we shall find the two extreme electroscopes indicating electrical activity, while the centre remains quiescent. Upon slowly withdrawing the excited body, these secondary electrical signs gradually decrease, and finally disappear upon its complete removal or discharge. Upon examining, by the proof-plane and torsion electrometer, the kind of electricity developed by this distant influence, we shall find that the end of the cylinder which is the nearest to the originally charged body is in an opposite state to that body, and the farthest end in the same state; that the cylinder has, in fact, had a polar state communicated to it (§ 15). This distant action of an electrified body, by which its own charge is in no degree lessened, is distinguished by the name of induction: the secondary state of the neighbouring body is called induced electricity and the body itself is said to be under induction. Farther, the originally active body is conveniently distinguished as the inductive body, and that under its influence as the inducteous body.(40)

(40) Let a represent an electrified ball, and b c an insulated metallic cylinder near it, the feather attached to the end, b, will be attracted, and show that it is in an opposite state to that of the charged ball. If de be an insulated cylinder, placed near the first, it will also show signs of electrical change, and the extremity, d, will be in an opposite state to the end, c, of the

214. If, while in a state of electric induction, that end of the cylinder which is most remote from the inductive body have its electricity discharged by a momentary contact with an uninsulated conductor, upon removal of the inductive body or its discharge, the cylinder will be found to be permanently electrified with the contrary electricity: or if the cylinder be divisible into two at its centre, and, while under induction, the opposite ends be separated from each other, one will be found permanently electrified vitreously, and the other resinously, upon removal from the influence of the originally charged body. In the theoretical language of Du Fay's hypothesis, the fluid of opposite name to that of the originally excited body is drawn to that extremity of the cylinder which is the nearest to it, while that of the contrary name is repelled to the greatest distance. When the latter is withdrawn and the induction destroyed, it cannot return to neutralize the former, which consequently remains in an active state.

This polar state may be excited in a long series of insulated conductors by induction, the intensity, however, of the forces decreasing rapidly as the distance from the originally-charged body increases. Throughout the system, the vitreous pole or end of one will be opposed to the resinous pole of anfirst cylinder, for the feather attached to the two will attract each other. Upon removing the ball, a, from the vicinity of the

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d

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first cylinder, or upon discharging it, all signs of electricity in the two cylinders will disappear.