INDUCTANCE AND CAPACITY 415 



and only the reactive component produces a turning moment. 

 Then 



.'. D = 2uKVIL x or L x = -~ (46) 



This method may be applied to the measurement of small 

 inductances having a large current-carrying capacity and a small 

 resistance ; for example, it has been applied to shunts such as are 

 used for alternating current measurements. 



The secondary of the air-core transformer may be the secondary 

 of an ordinary induction coil. The primary may be wound to 

 have a number of turns depending on the current to be dealt 

 with. 



The Measurement of Inductances Containing Iron. The pre- 

 ceding methods are adapted to the measurement of coils with air 

 cores, for in that case the self-inductance is constant, as has been 

 assumed in all the demonstrations. If the coils have iron 

 cores, then owing to the dependence of the permeability on the 

 degree of saturation of the iron, the self-inductance is no longer 

 constant but varies during the cycle. 



In such cases the effective inductance may be determined from 

 measurements of the applied voltage, current, frequency and 

 power. 



L - iV^F (47) 



The current should be adjusted to the value it has in the ordinary 

 use of the apparatus. With telephonic apparatus it is possible 

 to obtain satisfactory results by bridge methods, for the satura- 

 tion is so low that the permeability is practically constant. 



Measurement of Mutual Inductance. An obvious method of 

 determining the mutual inductance of two coils is to connect 

 them in series and to measure, by any convenient method, the net 

 self-inductance of the combination; then to reverse one of the 



