ELECTRICITY. 756 ELECTRICITY constant ^vhic■ll is characteristic of glass, and is called its "dielectric constant.' It can be shown easily that this constant is the K which occurs in the formula for electric action. K is said to measure the "induct ivity' of the dielectric. It may be proved from mathematical reason- ing that the capacity of an isolated sphere of radius R in a medium whose dielectric con- stant is K is KK; the capacity of a condenser of two parallel pUiles of area A and at a distance apart d with a dielectric of constant IC is KA/iTTd; etc. 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 retjuircs work to pull them apart. The electrical sci)aration, therefore, has an amount of energy equal to this work. This energy is present as a strain in the ether and dielectric which surround the charged bodies, as is shown by the fact that if the charges are too great the dielectric is ruptured — a sjjark passes. Since cliarges are always on the surfaces of conductors, it is evident that the material of a conductor yields to whatever stresses accom- l)any 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 at right angles to them. If a condenser has a charge +e and — e, and a dill'erence of iiotential Vj — V, Ijptween the con- ductors, the process of charging may be imagined as having bcgiui when there was no charge (and therefore no difference of potential), and that minute quantities of positive charges were car- ried from one plate to the other, leaving there- fore equal amounts of negative on the first plate. In this way the charges will gradually grow to +e and — e, and the difference of potential will increase at the same rate from to V, — V;, the average difference being therefore A(V, — Vj). The difference of ]>otcntial lictween the conduc- 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 charging the condenser a quantity of positive charge e has been carried across at the average difference of potential ifV, — V, the work required is e times I (V, — V,)' In other words, the energy of the charged condenser is "W = ^e(V, — V,). Expressed in terms of the ca- pacity, C = ,7 ^> this becomes W = i,^ V 1 — > 2 -L It has been shown that C varies directly as K, the dielectric constant, Avhieh is greater for glass or paper than. for air: therefore if in an ;iir-con- dcnscr the air could be replaced by glass or paper, keeping the charges unchanged, the energy would be diminished. It follows then, that, since changes take place in nature in such a way as to decrease the potential energy, the paper or glass would be 'attracted' in between the charged conductors. All ceases of electric attraction and repulsion may be explained in a similar manner. Electrtc Oscillations. If an isolated con- ductor is charged, there will be a definite distri- bution of charge over the surface in such a man- ner as to satisfy the law of electric action: but. if another conductor is brouglit near it. the dis- tribiition will be altered by the new forces due 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- tric oscillations'; that is, if the -f charge has been drawn 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 l)roducing electric oscillations is to bring close together two conductors and charge them op- positely until a spark passes; this has the ctTect of joinmg them with an extremely good conduc- tor. It may be shown that, as a result, the con- ductor which was charged positively becomes negatively charged, then positively, "etc.. each successive charge becoming less and less, initil finally e<iuilibrium is reached. This is what liap- Jiens in general when the two conductors forming a charged condenser, e.g. the two coalings of a Leyden jar. are brought so close together that a spark passes. (The analogy to a vibrating pen- dulum slowly coming to rest is complete.) As a result of this oscillatory discharge, the energy of the charges disappears, it being spent, in part, in producing disturbances in the surrounding dielectric of the nature of waves. These waves are in the ether, and are identical properties with those which affect the 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 Wirei.e.ss Telegraphy; Coherer. Electrical I.nstri^jients. Electric charge* may be produced by various machines which render the jirocesses of charging by contact or bv induction continuous and automatic. The com- monest forms are the frictional machine (see Electrical M.vchixes), the electrophorus (q.v.), the induction machine. Instruments have been devised to measure dif- ference of electric potential both absolutely and relatively. They are called electrometers, and are described under that title elsewhere. See Electro .meter.s. 1araday- Tires. A convenient mode of de- scribing electrical phenomena is due to Faraday, who pictured the electric field round charged bodies as filled with tubes whose sides were Tiiade by lines of force. These tubes join positive with negative charges, and are chosen of such a cross- section that where they meet a charged surface the open end of each tube incloses a unit charge. The ttd)e will have in general a varying eross- .section. .iepending on the paths of the lines of force, and where the electric force is the great- est the tubes are most numerous. ELECTROKIXHTICS. Fr.xnAMENTAL PiiExoMEXA. If a charged electrical condenser is discharged by joining its txyo conductors or 'plates' by a loiifi conducting wire, it is observed that there is no oscillation, but that the charges disappear and there are certain phenomena in and near the wire. The temperature of the wire is raised: and a magnet suspended, free to turn, near the wire an<l parallel to it, will be defiected so as to tend to .".land at right angles to it. In special cases, other phenomena may be observed. The whole
