of Electric Induction and Conduction. 355 



electricity came, negatively on the other side. The potential 

 gradually falls from A to B, i. e. the tension in the cord gra- 

 dually increases as we pass along it from A towards B. The 

 dielectric is in a state of strain represented by the stretching 

 of the elastics ; and it is tending to produce a countercurrent, 

 i. e. to drive the cord back again. 



§ 3. Now clamp the cord by means of the screw $, take off 

 Wj and leave the whole to itself for some time. Several 

 things may happen. 



1st. The buttons may remain exactly where they were, which 

 represents a dielectric of perfect insulating power : the jar will 

 in this case remain charged for any length of time, and the 

 whole charge can be got out of it at any time by unclamping 

 the cord. In the symbols of § 8, 



r = «o, ^ = 0, S| = ?!=...=«, / 1= / 2 =...=Q, a=0. 



As soon as the screw S is undamped all the buttons spring 

 back, overshoot their mean position, and execute a rapid series 

 of decreasing oscillations, carrying the cord with them back- 

 wards and forwards until they come to rest. The discharge of 

 a jar, then, is not a simple passage of electricity in one direc- 

 tion only, but is a rapid series of discharges in alternately op- 

 posite directions, which last until the energy stored up as strain 

 in the dielectric is all converted into heat by the resistance of the 

 circuit. This succession of alternately opposite sparks Fedder- 

 sen* has observed experimentally by looking at the apparently 

 simple spark of a Leyden jar in a revolving mirror. If it 

 were possible to close the circuit so rapidly that very little of 

 the discharge should have taken place before good metallic 

 contact was made, the succession of opposite currents would 

 probably last quite an appreciable time. They might perhaps 

 be detected by discharging the jar through a galvanometer and 

 an electrodynamometer at the same time ; the relation between 

 their indications would probably be different in this case from 

 what it is when either a continuous current or a simple one- 

 directioned discharge is sent through them both. 



2nd]y. The buttons may all slowly and equally slide back 

 towards their normal position. This will happen if the relation 

 between their friction on the cord and the elasticity of their 

 strings is the same for all. The model will in this case repre- 

 sent a homogeneous dielectric with feeble cond acting-power. 



* Pogg". Ann. vol. ciii. Fecldersen saw the spark drawn out into a suc- 

 cession of "bright parallel lines at continually increasing- intervals. Max- 

 well's theory, equation (2), § 9, appears to make the intervals all equal, but 

 only on the erroneous assumption that the circuit is devoid of resistance. 



2 A2 



