ELECTRICAL MEASUREMENT 



83 



Fig. 8. It may be seen from this figure that all capacitances which 

 include dielectric material are permanently connected across CD or 

 AC and so are not changed when the condenser is switched, or else 



D IC 



A 



Fig. 8 — Air-condenser construction employed in the capacitance and conductance 



bridge 



they are switched so that capacitances across A C, which do not enter 

 into the bridge balance, are short-circuited on switching. This scheme 

 eliminates all dielectric loss in the standards when measuring con- 

 densers by comparison with them. It has the additional advantage 

 that the capacitances in the bridge have twice the effect they would 

 have if simply switched in and out of the circuit. 



By the use of this bridge, it is possible to measure capacitances up 

 to the maximum limit of the range of the air condensers with a negli- 

 gible loss in the standard condensers. This capacitance range is 

 usually up to 0.01 (jl f. and for condensers above this value the con- 

 ductance is measured by comparison with that of the maximum value 

 of the air condenser, assuming it to have negligible conductance. Of 

 course this method of eliminating dielectric loss is not applicable to 

 the use of mica condenser standards and if a range greater than 

 0.01 ju f. is desired, the mica condensers are simply connected in the 

 usual way across AD. 



Another feature of this bridge is the method of measuring con- 

 ductance. The connection of a variable resistance, either in series or 

 in shunt, with the standard condenser for the measurement of loss 

 in the test condenser has objections due to the wide range of resistance 



