420 silsbee: inductance of resistance standards 



did not in general satisfy the ideal condition of giving a secondary 

 voltage in exact quadrature with the primary current but that 

 the voltage had a small in-phase component. By analogy with 

 the case of the shunt we may define the "resistance" of the mu- 

 tual inductance as the ratio of this in-phase component of the 

 voltage to the primary current. We will further define the 

 "phase defect" as the angle whose tangent is the ratio of the 

 in-phase component of the voltage to the quadrature component. 



The method which was found most suitable for the comparison 

 of the time-constants of two shunts may be called the current 

 transformer method. It consists essentially in measuring the 

 apparent phase angle of a current transformer by one of the 

 usual null methods, using in succession, as the standard resist- 

 ance in the primary circuit of the transformer, the two shunts 

 to be compared. The apparent change in the phase angle of 

 the transformer is the difference in the phase angle of the two 

 shunts. A group of about twenty shunts were intercompared 

 by this method and form a basis for future comparisons. A 

 second method involving the use of mutual inductances was 

 tried out, and gave results in agreement with the first method, 

 but was found to be much less convenient. 



Since the comparison methods just mentioned give only the 

 difference in time constants of two four-terminal standards, 

 some other measurement is needed to give the actual value of 

 the time-constant of one four-terminal shunt in terms of known 

 quantities. The simplest way to obtain this value is to construct 

 a shunt of such shape that its inductance can .be computed from 

 its measured dimensions. This procedure requires that cer- 

 tain assumptions be made as to current distribution, etc.; a 

 careful investigation using three different shapes of shunt showed 

 that the assumptions made are completely justified. As a check, 

 two other methods of measurement were tried; one involved 

 the use of mutual inductances of known phase defect, and the 

 other made use of two shunts constructed of identical dimensions 

 but of materials of different resistivities. These methods gave 

 results in agreement with the computed values, but were less 

 accurate and more laborious. It is believed that the time con- 



