356 ELECTRICAL MEASUREMENTS 



A is the total active area of one set of plates and t is the thick- 

 ness of the dielectric. 



To transfer the value of the capacity from the electrostatic 

 to the electromagnetic system of units, 



v = 2.996 X 10 10 

 v z = 8.976 X 10 20 . 



One c.g.s. electromagnetic unit of capacity = v 2 c.g.s. electro- 

 static units of capacity. 



One c.g.s electromagnetic unit of capacity = 10 15 microfarads. 



Consequently the capacity in microfarads of a parallel plate 

 condenser is 



(2.996 X 10 10 ) 2 4xrt ~ IWt ' 



Obviously, to increase the capacity without an undue increase 

 of bulk, a dielectric having a high dielectric coefficient and capable 

 of being used in very thin sheets must be employed. The 

 material chosen should have an exceedingly high specific resistance 

 and high dielectric strength. 



The above formula for capacity is convenient for rough pre- 

 liminary calculations; as t cannot be known with any great 

 certainty all condensers with dielectrics of small thickness must 

 be calibrated. 



All materials used in the construction of condensers must be 

 clean and perfectly dry and the finished instrument must be 

 sealed in some manner so that the access of moisture is prevented. 

 Mica and paraffined paper are the materials commonly used 

 for the dielectric. Air pockets in the dielectric must be entirely 

 eliminated. 



Temperature has an appreciable effect on the behavior of 

 condensers having solid dielectrics. It is not possible to give 

 a definite statement as to the temperature coefficient of the 

 capacity of a particular condenser, for the temperature effects 

 are dependent on the particular cycle of operations to which 

 the condenser is subjected. This is illustrated in Figs. 212A and 

 212J5, which are typical of good and poor mica condensers. The 

 best mica condensers when subject to the ordinary fluctuations 

 of room temperature may show variations in the capacity of 2 

 or 3 parts in 10,000. 



