150 



Mr. R. Appleyard on Dielectrics. 



again applying the current and noting the deflexion after 

 one minute. 



It sometimes happens that we want to separate (a) from 

 (b) and (c) ; that is to say, we require to know what part of 

 the preliminary deflexion is due to the exposed lead, and what 

 part is due to internal leakage at the condenser. For this 

 purpose I have applied a suggestion made to me by Mr. W. 

 A. Price *, and have used a " guard-wire " to eliminate the 

 leakage at (a). The method, which was used for its first 

 trial in connexion with these experiments, consists in winding- 

 one or two turns of bare wire around the cleaned end of A, 

 at some point not very far from the middle of the cleaned 

 portion, intercepting the path of leakage. This wire is then 

 connected to the testing-battery, as shown in Fig. 1. 



Ksr. 1 



9 



-0- 



7n n 



Guard-wire 



The efficacy of the " guard-wire " for the present tests 

 was proved by the following experiment. A copper earth- 

 wire was given several turns around the insulated lead A, 

 just above the cleaned ends, to facilitate leakage ; and the 

 preliminary test was then made in the usual way. The 

 observed deflexion, due to (a), (b), and (c), was 4 divs. The 

 cleaned end was then smeared with a mixture of glycerine 

 and blacklead, and the resulting deflexion was far off the 

 scale ; (a) had been increased enormously. A "guard-wire " 

 was then applied at the middle of the smeared portion of the 



lead, and the deflexion was at once restored to its 



oris 



inal 



value of 4 divs. 



In practice it was always possible to avoid leakage (a) by 

 giving a little care to the cleaning of the lead ; the " guard- 

 wire " was therefore used only as a check, and if it produced 

 no change in the preliminary deflexion it was concluded that 

 the preliminary deflexion was due to the effects of (b) and (c) 

 alone, and that there was no leakage (a). 



* This method has since been described by Mr. Price. See ' Electrical 

 Review/ vol. xxxvii.no. 941, p. 702(1895). In the appendix to the 

 present paper I have evaluated the possible errors, and shown that they 

 are negligible. 



