Mr. R. Appleyard on Dielectrics. 149 



and eight mica condensers, each of half a microfarad, were 

 enclosed, separately, in water-tight cases, and submerged in 

 a tank of water, the temperature of which could be con- 

 tinuously controlled. The platinum thermometers * used for 

 determining the temperature within the dielectric of the 

 condensers have already been described. 



Connexion between the condensers and the testing- 

 apparatus was made by wires insulated with india-rubber ; 

 these were twenty-five feet long. The connexions within 

 the condenser cases, where the wires were soldered to the 

 respective sets of tin-foil sheets, were carefully sealed with 

 paraffin wax. The ends of the leads were bared and cleaned 

 in the usual way, for about six inches. Two such insulated 

 wires were connected to each condenser, one to each set of 

 tin-foil sheets. 



Surface Leakage. 



Suppose a current to enter one of these condensers at one 

 of its insulated leads, which we will call A ; and the second 

 lead, B, put to earth. It is clear that there are at least four 

 paths — (a), (6), (c), (d) — for the current between A and 

 earth : — 



(a) Surface leakage at the end of A where the current 



enters the lead. 



(b) Surface leakage at the end of A within the condenser. 



(c) Surface leakage over the dielectric within the con- 



denser. 



(d) Through the dielectric of the condenser. 



The leads can generally be so chosen that there is practically 

 no conduction through their dielectrics. When measuring 

 the resistance of the condenser by the " direct deflexion " 

 method, the current to which the galvanometer responds is 

 that due to the sum of the currents through (a), (b), (c), and 

 (d). We have to determine what the deflexion would be if 

 all the current went by way of (d). This can be done, 

 approximately, by taking a preliminary test, with the end B 

 disconnected from earth, and carefully cleaned ; noting the 

 deflexion after the current has been applied at the end A 

 for one minute. The deflexion thus obtained must be 

 deducted from the deflexion observed during the second 

 operation, which consists in putting the end B to earth, and 



* A " Direct-reading " Platinum Thermometer. Proc. Physical Soc. 

 March 1896 ; and Phil. Ma?. Jan. 1896. 



