THE DIELECTRIC 113 



RESIDUAL CHARGE AND DISCHARGE 



The investigation of specific inductive capacity is very much 

 complicated by what are termed residual effects. If a Ley den 

 jar is charged and, after standing for a short time, is discharged by 

 a spark in the ordinary way, it appears to be completely discharged 

 and the two coatings are at the same potential. But if the jar is 

 allowed to stand for a short time, with the inner coating insulated, 

 a new charge gathers of the same sign as the original charge, and a 

 second much smaller spark may be obtained on discharging it. 

 This process may be repeated, and with some jars three, four, five, 

 or more visible sparks may be obtained in succession, the jar being 

 allowed to rest insulated after each discharge. If, immediately 

 after the first discharge, the inner coating is connected to a gold- 

 leaf electroscope, the charge can be seen to gather, for the leaves 

 diverge till they touch the side plates and so discharge the jar. 

 They will diverge and discharge many times in succession. The 

 electroscope may be used to show that all these charges are of the 

 same sign as the original charge. They are known as Residual 

 Charges, and the discharges as Residual Discharges. 



If a jar which shows very conspicuous residual effect is charged 

 to some measured potential and left insulated, it is found that the 

 potential gradually falls, and the discharge obtained on connecting 

 the coatings for a moment will be less than the original charge. 

 This fact suggests that the phenomenon is in some way connected 

 with conduction, and Faraday sought to explain it by supposing 

 that the two charges left the plates to some extent and penetrated 

 the dielectric towards each other, some of the + charge on 

 AB, Fig. 79, for instance, reaching ab, while some of the 

 charge on CD reached cd, ab and cd being probably further in 



Fio. 79. 



the longer the time.. On discharging AB and CD the charges 

 ab and cd would no longer be pushed from behind by charges 

 on the plates, and some part of them would return to AB and 

 CD and be ready for a second discharge. But when we come to 

 examine the process of conduction we shall see that this account, 

 though probably containing a good deal of truth, hardly gives the 

 correct view as it stands.* 



R. Kohlrauschf showed that if a given jar is charged to a certain 

 * Exp. Res., vol. i. 124:,. f ?W> Ann. xci. (1854). 



n 



