ELECTRICAL MEASUREMENT 



73 



of the bridge are such that the value of each capacitance may be 

 determined in terms of the frequency and the two resistances. 



Fig. l^Bridge circuit for measuring capacitance in terms of resistance and frequency 



The requirements for a capacitance standard are high constancy 

 with variations in frequency, time, voltage, and atmospheric condi- 

 tions, and a small phase difference. Mica has been found to be the 

 best solid dielectric, used either alone or impregnated with a high 

 quality wax such as paraffin. If mica alone is used, the condenser 

 must be sealed to prevent the entrance of moisture. 



Good mica condensers can be obtained with a temperature coeffi- 

 cient below 0.005 per cent per deg. cent., and having a variation 

 of less than 0.1 per cent over a frequency range from 500 cycles to 

 100 kc. Variations in capacitance with voltage are also negligible 

 provided voltages below 100 volts are used. It has been our experi- 

 ence that the paraffin-impregnated condensers generally have a nega- 

 tive change of capacitance with temperature. This change is smaller 

 than that of the unimpregnated type which has a positive change with 

 temperature. The paraffin-impregnated condensers, however, usually 

 change more with time than the unimpregnated condensers. 



Air condensers may be used as standards in small sizes. For the 

 larger values, the air condensers become large and cumbersome and 

 are not as stable as the mica condensers. Even in the smaller sizes, 

 very special precautions must be taken to obtain air condensers which 

 have appreciably smaller phase differences than the mica condensers, 

 which may be made with phase dififerences considerably less than one 

 minute. 



Inductance. Requirements for inductance standards are high con- 



