Theory of Induction.—Faraday, taking for granted that the dielectric is the essential medium of induction, suggests that the molecules of air and other dielectrics are con -ducting, but that they are insulated from each other. We have already seen that by induction, part of the E. of an insulated body can be in effect transferred to a surface at some distance from it, without any loss experienced by the exciting body. If, now, we could imagine a series of insulated cylinders diverging in all directions from the .glass tube, we have reason to expect that the whole of the E. of the tube would be in •effect transferred to their outer extremities without loss of E. to the tube. To prove that such would be the case, Faraday took a pewter ice-pail, 101 in. high, and 7 in. in diameter, and insulated it, placing the outside of it in conducting connection with the knob of a gold-leaf electroscope. An insulated ball, charged with +E., was then intro .duced into it without touching. The pail was thus subjected to polarization, the — E. being on the inner, and the + E. on the outer surface. The divergence of the leaves increased as the ball was lowered, until it sunk 3 in. below the opening, when they remained steadily at the same points. The ball was lowered till it touched the bottom, and communicated its charge to the pail, when the leaves remained in the same state as before, showing that the + E. developed by induction on the outer surface was exactly the same in amount as that of the ball itself. IIe then altered the experiment so as to have four insulated pails inside 'each other, and the effect on the outmost pail was in no way altered. Here the action of the air between the pails was in effect the same as that -of the pails themselves, and if the molecules of air were insulated conductors like these, they would have acted in no way different from what they did. The action of the molecules of air, in certain circumstances, appears to favor the idea that they are indi vidually conducting. The discharge of E. by spark through the air, shows that they can be forced to act as conductors; and the currents which proceed from points highly charged with electricity, appear to indicate that they can be attracted and repelled like the pith-balls of our first experiment. • Conductors, according to this theory, are bodies whose molecules have the power of •communicating their electricities to each other with great ease, whilst non-conductors .are those whose molecules only acquire this power under great force. Wheatstone has .shown, as we shall afterwards see, that facility of discharge is not perfect even in the best conductors, as time is needed for its propagation, and it has been found that the terminal laminm of non-conductors between two charged plates become penetrated with opposite electricities, which indicates the slow progress of conduction. The molecules of conductors and non-conductors, therefore, have the same power of mutual discharge, but in very different degrees, so that a good non-conductor may be regarded as an excessively slow conductor.
Potential, Density, Tension, Capacity.—Some idea of the meaning of the word potential may be got from the following comparison. Suppose we have a supply of water with .a certain head, to fill an bag: when the water is admitted, the bag will swell till the elasticity of the bag is equal to the head of water, and then the flow will cease. The potential is the head of water or elasticity of the hag, so many feet high, or so many pounds per square inch. The capacity of the bag is usually the amount it holds, but capacity in an elastic bag is a shifting quantity, and we must use the term in this way if we wish to compare the capacity of two elastic bags—viz., the ratio of the water it. holds to the head that filled it. Thus, a bag holding 10 galls., with a head 1 ft., would have a 10 times greater capacity than a bag holding 10 galls. with a head 10 ft.; for the first were pressed by a head of 10 ft., it would hold 100 galls., the resistance of the
bag being supposed to increase with its contents. Now, let us take a somewhat similar electric problem. An insulated ball is connected with a magazine of energy, ready to. make E. flow when occasion offers, such as a galvanic battery. Let the + pole of a gigantic battery be connected with the ball, and the other pole with the ground, E. will flow to the ball till the air between the ball and the ground presents an electric reaction equal to the potential of the battery. The charge of the ball taken with reference to this potential gives the capacity of the ball. So much, then, for a popular view of these. two words. The potential of a body, or any point in the field, is defined thus—viz., the amount of work that-would be expended in bringing a small quantity, a unit of + E., from an infinite distance to the body or point. If the body is positive, the work would be expended; if negative, the work would be done on the body and the potential —. The said unit of + E. will always move from a point where the- potential is high to one where it is lower; in other words, E. will always flow between two points where there is a difference of potential, and will cease to flow when that difference ceases. If E be charge, V the potential, C the capacity, then = E V. From the definition of potential just given, what we have called the potential of the battery in the preceding illustration is in reality its electromotive force, or the difference of potentials of its poles. As these are alike in power, but different in sign, and as the difference of two quantities. of unlike sign is their sum, the electro-inotive force is twice the potential of one pole. If the charging line be withdrawn, the ball will be in all respects as if charged by an electric machine. The battery having, so long as it acts, an unlimited supply of E., its electro-motive force remains the same; but when balls charge one another, the potential falls just as when a limited supply of water has its head reduced when made to run into another vessel. Potential, then, must be estimated by the resistance of the or the work value of the unit of charge. The charge being the same, the potential rises with the smallness of the body, or the thickness of the dielectric. Density is the quantity of electricity on a unit of surface, and tension is the strain which Faraday supposes to exist in the molecules of a dielectric when charged. Tension is commonly used in this coun try and abroad for potential, though our best writers never use it now in this sense.
Distribution of might take it almost as a self-evident truth, that the. greater the surface over which E. is diffused, the less is its electric potential at any par ticular point, and so we are taught by experiment. When two equal balls are insulated, and a charge is given to one of them, and then communicated to the other by contact with the first, it is found that both equally divide the charge, but that the potential of the E. of each is one half of that of the originally charged ball. When a watch guard chain is charged and laid on the plate of an electroscope by means of a glass rod, the• gold leaves diverge most when the chain lies in a heap on the plate; and as it is lifted up, the leaves approach each other, showing that as the exposed surface of the chain increases, the electric potential of each part diminishes. The reason of this is obvious. Let us begin with one ball with a certain charge, then take another equal ball and impart half the charge to it by making the two touch. A spark will be seen at the charge of the second ball. The quantity in both is still the same, but energy has been lost by the spark, and the heat generated by the spark is the measure of the loss. If we continue to add ball after ball until we have a very large surface, the quantity is the same as at first, but energy has been squandered in the sparks of each additional ball, and so the potential is lowered.