INDUCTION INSTRUMENTS 451 



T = KJ* ^p [sin (fa - fa)] (2) 



fa - fa = 90 approx. 



The torque and therefore the deflection are proportional to the 

 mean square of the line current. 



Frequency and Temperature Effects in Westinghouse Ammeter. 

 In order that an instrument may be of the highest utility, its 

 indications must be free from the effects of change of frequency 

 and of temperature. Practically, these effects must be reduced 

 to amounts which are consistent with good engineering practice, 

 since they cannot be made zero. As Z, 7, and (fa Pi) vary 



cos 7 sin (fa fa) 



with the frequency, the factor - -^ can be made 



z/ 



only approximately constant. By careful design, the maximum 

 difference in the readings corresponding to a definite current, 

 for any two frequencies between 25 and 60 cycles, may be reduced 

 to about 0.5 per cent. 



In order that the temperature effect be nil, for any definite 



current, s ~ p cos 7 must be constant with respect to temperature. 

 & 



This relation is readily attained as the arrangement is electrically 

 equivalent to a current transformer. It is to be remembered 

 that in a current transformer if the primary current be kept 

 constant, the secondary current will be practically unaffected 

 by considerable changes in the resistance of the secondary circuit. 

 This means that the mutual flux, $P, increases in proportion to 

 the secondary resistance. The flux 3> s is due to the secondary 

 current and is, therefore, fixed in value; consequently, if by the 



use of the proper materials in the secondary, <p be given a tem- 



r^ 



perature coefficient equal to that of - , the required adjust- 



cos 7 



ment is attained. Therefore, the secondary circuit is made 

 partly of copper and partly of resistance wire of low temperature 

 coefficient, the proper proportion of the two being determined 

 experimentally, so that allowance can also be made for the 

 temperature coefficient of the iron and of the controlling spring. 

 In the voltmeter the primary coil is wound with fine wire, and 

 an external non-inductive resistance wound with wire of zero 

 temperature coefficient is used. The proportion of ohmic re- 



