IN THE SOLID AND LIQUID PHASES. JOHNSTONE. 195 



At the instant t, the difference in potential between the 

 two pairs of quadrants is: 



i- Cos. tot A- Cos. (t + <&). 

 Ci <C 2 



If < = and if the above potential difference be zero, 

 then there will be no deflection of the electrometer needle. 

 Then: 



Ii _Cl 

 I 2 " C 2 

 that is, CiRi= C 2 R 2 



which is the condition for no deflection. Hence if Ci, RI and 

 C2 be so chosen that with a given R 2 , no deflection of the 

 electrometer results, it is possible from the above equation to 

 obtain the value of R 2 . It is well to remark, that the most 

 sensitive conditions for a balance exist when the reactances of 

 condensers Ci and C 2 are respectively equal to the resistances 



C* C* 



RI and R 2 . In practice if the ratios - and - are each 



RI R 2 



between 5 and \ comparatively good working conditions 

 exist. This method is given in some detail, as it may be set 

 up with ease and it will give very satisfactory results. Further- 

 more, it does not involve any knowledge regarding the abso- 

 lute value of the capacities. It is the writer's belief that it 

 deserves more general attention than it has hitherto received. 



A standard ^M. F. condenser, manufactured by Leeds 

 and Northrup, was used for capacity Ci. 



The reactance of this condenser is 8000, approx., for a 

 60 cycle E. M. F. Four adjustable resistance boxes, manu- 

 factured by the same firm and having a combined resistance 

 of 40,000 ohms were used for R]. The values of R 2 , which 

 were measured by this method, ranged from 10 5 to 10 8 ohms 

 and as considerable accuracy was desirable, it was necessary 

 to manufacture three capacities having approximate reactance 

 values of 10 6 , 10 7 and 10 8 respectively. Two mica condensers 



