INDUCTION INSTRUMENTS 



455 



must be altered and another trial made, and so on until the 

 results do agree. 



As the correct action of an induction wattmeter depends on 

 having the angle A exactly 90, it is obvious that a change of 

 frequency, with its accompanying change of reactance of the 

 potential coil, will introduce errors. An induction wattmeter 

 which is adjusted for use on a 60-cycle circuit will be in error if 

 used at 25 cycles. As the correctness of the instrument is de- 

 pendent on the frequency, it will be affected by changes of wave 

 form. See the discussion of the induction watt-hour meter, page 

 473. 



In another lagging arrangement, not commonly used in 

 America, the potential coil is shunted by a non-inductive resis- 



FIG. 264. Diagram for lagging arrangement for induction wattmeter. 



tance and this divided circuit placed in series with a reactance. 

 This whole combination forms the potential circuit of the 

 instrument. 



This arrangement is indicated in Fig. 264. 



The equations are: 



IAB = In + IP 



v =y AB + y BC , 



By altering the non-inductive resistance, R, the component, 

 I R , may be varied and the phase of I P , or more correctly, the phase 

 of $p, which will differ a little from that of I P , may be changed 

 until A = 90. 



In the induction wattmeter, temperature changes affect the 

 resistance of the rotor or movable element; the induced currents 

 are thus cut down about 0.4 per cent, per degree of temperature 



