58 ALTERNATING CURRENTS 



whore P' is the power indicated by the wattmeter, / is the current 

 in the wattmeter current-coil, and R c is the resistance of this coil. 

 This loss is ordinarily of the magnitude of 1 or 2 watts at the 

 rated current of the instrument, and may often be neglected. 



If the wattmeter be connected as shown in Fig. 54 (6), the 

 wattmeter potential-circuit is connected directly across the load, 

 but the wattmeter current-coil carries the potential-coil current in 

 addition to the load current. In fact, the wattmeter potential- 

 circuit may be considered as being a small load connected in 

 parallel with the actual load whose power is to be measured. 

 Therefore, the power consumed by this potential-circuit must 

 be deducted from the wattmeter reading. The true power taken 

 by the load, 



P = P' - E*/R P 



where P' is the wattmeter reading, E the load voltage and R p the 

 resistance of the wattmeter potential-coil circuit. 



An idea of the magnitude of this correction may be obtained 

 from the following example. 



Example. A certain wattmeter indicates 157 watts when it is connected 

 in the manner shown in Fig. 54 (b). The line voltage is 120 volts and the 

 resistance of the wattmeter potential-circuit is 2,000 ohms. How much 

 power is taken by the load? 



P = 157 - 120 2 /2,000 = 157 - 7.2 = 149.8 watts. 



It will be observed that a considerable percentage error would 

 result in this case if the wattmeter loss were neglected. 



The Weston Electrical Instrument Co. manufactures an 

 instrument which compensates for this loss. A small auxiliary 

 coil, connected in series with the moving-coil system, is inter- 

 wound with the fixed coils so that a small counter-torque is 

 exerted, this counter-torque being proportional to the power 

 consumed by the potential circuit. 



The current- and potential-circuits of a wattmeter must each 

 have a rating corresponding to the current and voltage of the 

 circuit to which the wattmeter is connected. A wattmeter is 

 rated in amperes and volts, rather than in watts, because the 

 indicated watts show neither the amperes in the current-coil nor 

 the voltage across the potential-circuit. 



If the current in an ammeter or the voltage across a voltmeter 



