204 



ELEMENTS OF ELECTRICAL ENGINEERING. 



circuit AR. The compensated wattmeter of the Weston Elec- 

 trical Instrument Company is designed to eliminate these sources 

 of error. This wattmeter is connected as shown in Fig. 122, but 

 the wire leading to the coil A is carried back through the coil B 

 and laid alongside of the windings of B as shown in Fig. 123. 



Let / be the current flowing through Z, Fig. 122 ; let E be the voltage between 

 the terminals of Z, and let i be the current flowing through A and I\. Then the 

 current flowing through B is (/-J- z'), and the force action between the two coils is 

 proportional to z X (^+ *') By laying the fine wire along side of the coarse wire in 

 coil B, as shown in Fig. 123, the electromagnetic action of coil B is reduced to what 

 it would be if the current /, only, were flowing in it, so that the force action between 

 the two coils in Fig. 123 is proportional to z'X^> or to E\Ry^I, or to El. That is, 

 the force action is proportional to the power delivered to L so that the scale may be 

 arranged to indicate this power correctly. 



92. The watt-hour meter is an instrument for summing up or 

 integrating the total work or energy delivered to a circuit. It is 

 used chiefly as a meter for recording the amount of energy de- 

 livered to a customer by a central station. The Thomson watt- 



s 



Fig. 124. 



hour meter of the General Electric Company is suitable for both 

 direct and alternating current service ; the induction watt-hour 

 meter is suitable only for alternating current service. The induc- 

 tion watt-hour meter is described in a later volume of this text. 



