ENERGY. Since when a body is charged by bringing it in intimate contact with another body, e.g. glass with silk, and then separating them. one is charged positively and the other negatively, it requires work to pull them apart. The electrical separation. therefore. has an amount of energy equal to this work. This energy is present as a sintin in the ether and dielectric which surround the charged bodies, as is shown by the fact that if the Charges are IGo great the dielectric is ruptured—a spark Since charges are always on the surfaces of conductors. it is evident that the material of a conductor yields to whatever stresses accom pany charges, and so cannot be under a strain. Faraday has shown that the strain in the dielec tric is of the nature of a tension along the lines of force and a pressure it right angles to them. If a condanser has a charge -f-c and —c, and a differellets Of potential h•tween the con. ductors. the process of charging may be imagined as having begun When there was no charge (and therefore no difference of potential), and that minute quantities of positive eharges were car ried front one plate to the other, leavim• there fore equal amounts of negative on the first plate. In this way the charges will gradually grow to and —c, and the difference of potential will increase at the same rate front 0 to the average difference being The difference of potential between the minim tors at any instant is by definition the amount of work required to carry a unit positive charge from one plate to the other; therefore, since in the process of eharging the condenser a quantity of positive charge .• has been carried across at the average differenee of potential the work required is c times I In other words, the energy of the chary-gl condenser is W.= • Expressed in terms of the ca racily, C= this becomes W = gas been shown that t' varies directly as K, the dieketrie constant. %vhich is greater for glass or paper than for air: therefore if in an air-con denser the air could be replaeed by glass or paper. keeping the charges unchanged. the energy would be diminished. It follows then. that, slime elianges take place in nature in such a way as ter decrease the potential energy, the paper or would be 'attraeted' in between the charged condnetors. All eases of electric attraction and
repulsion may be explained in a similar manner.
Eim•ron. nsctit..vrioNs. If an isolated eon du•tor is there will he a definite distri bution of I .harge over the surface itt suet a Man ner as to satisfy the law of electric• action: hut, if another conductor is brought near it. the dis trilmtion will be altered hy the new force: dine to induction. If now this second conductor is suddenly removed, or if the forces of induction are suddenly destroyed. the charge will return to its original distribution, Before doing so, however, there will be what may be called 'elec t ric oscillations': that is, if the + charge has been draw" toward one end of the conductor by the forces of induction. then when these are re moved it will appear at the farther end, next return toward the first end. etc.. gradually com ing to its state of equilibrium. These oscilla tions are immensely rapid, and generally die away extremely rapidly. The simplest mode of producing electric oscillations is to bring close together two conductors and charge them op positely until a spark passes: this ha s the effeet of joining them with an extremely good (.011(111(. 1w% It may be show-n that, as a result, the con ductor which was positively becomes negatively ehargeti. then positively, etc., each successive eliarge becoming less and less, finally- equilibrium is reached. This is what hap pens in general when the two conductors forming a charged condense,-, e.g. the two coatings of a Leyden jar. are brought so close together that a passes. (The analogy to a vibrating pen dulum slowly coming to rest is complete.) .1.s a result of this oscillatory discharge. the energy of the charges di,appears, it being spent. in part, itt producing disturbances in the surrounding dielectric of the nature of wages. These 'waves are in the ether, and are identical properties with those which aireet Ilit• sense of sight or tem perature. being, however, longer, i.e. of a smaller wave-number. They do not appeal to any human senses, but may be detected by various physical means. See WIRELESS TELEGIL‘PlIY; Conti:ha.