IN THE SOLID AND LIQUID PHASES. — JOHNSTONE. 195 



At the instant t, the difference in potential between the 

 two pairs of quadrants is: 



— Cos. «t — -li- Cos. («t + <I>). 

 «Ci (0C2 



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



then there will be no deflection of the electrometer needle. 



Then: 



h _Ci 



I2 ~ Co 



that is, CiRi= C2R2 



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



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



electrometer results, it is possible from the above equation to 



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



sensitive conditions for a balance exist when the reactances of 



condensers Ci and C2 are respectively equal to the resistances 



C Co 



Ri and R?. In practice if the ratios — - and -^ are each 



Ri Ro 



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. iFurther- 

 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 iM. 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 Ri. The values of Ro, which 

 were measured by this method, ranged from 10^ to 10^ ohms 

 and as considerable accuracy was desirable, it was necessary 

 to manufacture three capacities having approximate reactance 

 values of 10^ 10" and 10* respectively. Two mica condensers 



