360 ELECTRICAL MEASUREMENTS 



Then 



r s = 120 ohms r p = 75,000 ohms 



leakance = 0.0000133 + 

 C s = 0.8848 microfarad C p = 0.8835 microfarad 



In cases where the dielectric losses are large, the equivalent 

 capacities for the series and the parallel arrangements are slightly 

 different. 



The equivalent parallel resistance r p has no relation to the 

 insulation resistance as measured with direct current. 



Mica Condensers. Mica is used as the dielectric in the best 

 working standards of capacity. Its. specific resistance is about 

 1 X 10 10 (megohm, centimeter). Its dielectric coefficient 

 varies between 6 and 8. The puncturing voltage of selected 

 specimens 0.1 mm. thick, when tested between plates may be 

 as high as 12,000 volts (r.m.s.). The average strength is much 

 lower. 



Mica condensers are not ideally perfect and vary greatly in 

 their properties, so that in careful work the characteristics of 

 the particular condenser employed as a standard must be known. 

 However, a mica condenser always behaves in the same manner 

 if the same conditions be maintained. For this reason, in work 

 of high precision, the cycle of operations to which the condenser 

 is subjected, both when its capacity is determined and in sub- 

 sequent use, should be the same. The capacity of a good mica 

 condenser is independent of the voltage. 



To be useful as a standard a mica condenser must be firmly 

 clamped. 



Experiments show that the capacity of a good mica con- 

 denser, when determined at higher and higher frequencies by a 

 method of rapid charge and discharge using direct currents, 

 approaches the same value as that .obtained by the use of alter- 

 nating currents, the period with alternating currents and the 

 time of discharge with direct currents being the same. The two 

 curves connecting the reciprocal of the frequency and the capa- 

 city and the time of discharge and the capacity, when extrapo- 

 lated for infinite frequency and zero time of discharge apparently 

 cut the capacity axis at the same point. The capacity determined 

 by this process of extrapolation is called the " instantaneous," 

 or by some writers, the " geometric" capacity, being independent 



