480 



ELECTRICAL MEASUREMENTS 



it may be necessary to insert a thermometer in the instrument 

 in such a manner as to give the mean temperature, and to pro- 

 vide a calibration card which will give the necessary corrections 

 for the ordinary range of atmospheric temperature. 



Fig. 273 shows the effect of change of temperature on an induc- 

 tion watt-hour meter of accepted design. 



Frequency Errors. The effect of a departure from the normal 

 frequency may be shown qualitatively as follows. 



Suppose that the frequency is doubled, the voltage, current, 

 and power factor of the load remaining fixed. Assuming that 

 the resistance of the potential coil is small, the potential-coil flux 

 will be halved. However, the currents induced in the disc by this 



1.08 

 1.06 

 1.04 

 1.02 

 1.00 



P.F.- 



Induction Watt-hour Meter 



Effect of Frequency and Overload 



Smooth Waves. 



50 Cycles 



60 Cycles 



70 Cycles 



110 Volts 



70 80 90 100 110 120 130 140 150 ICO 170 IbO 190 200 

 Per Cent K.V. A.Load 



10 20 30 40 50 



FIG. 274. Showing effect of frequency on induction watt-hour meter. 



flux will remain as before, for, while the flux is only one half 

 as great, it is varying at twice the frequency. The induced cur- 

 rents react with the current-coil flux, which is fixed. The net 

 result is that the alteration in this portion of the accelerating 

 torque is that due to the changed time-phase relation of the fluxes. 

 The currents induced in the disc by the current-coil flux are 

 doubled, for though the value of th6 flux is not changed it is 

 varying at twice the normal frequency. These doubled currents 

 react with the halved potential-coil flux, so again the effect is 

 that due to the changed time- phase relation of the fluxes. 



