H 8 TRANSFORMERS. [Exp. 



resistance of the potential coil of the wattmeter. This correction 

 might be, for example, 5 watts in a loo-volt wattmeter, 10 watts in a 

 2oo-volt wattmeter, etc. The correction, however, is exact and is 

 readily made, the value of R w usually being given with the instrument. 

 In precise work, this method should be used and the correction made. 

 If, however, no correction is to be made, it is better to use the first 

 method, in which the error is smaller. 



40. Use of a Voltmeter with a Wattmeter. When a voltmeter 

 and wattmeter are used together, the voltmeter should (usually) be 

 connected between the same points as the potential coil of the watt- 

 meter. There are, therefore, the same two methods of connection as 

 with a wattmeter alone. 



41. First Method. The voltmeter is connected between the lines 

 on the supply side of the wattmeter. The reading of the voltmeter 

 includes the RI drop in the current coil of the wattmeter; the error is 

 small and may often be neglected. 



42. Second Method. The voltmeter is connected on the load side 

 of the wattmeter, directly to the terminals of the load. The volt- 

 meter reading is now correct. The wattmeter, however, includes the 

 watts consumed in the voltmeter. The reading of the wattmeter 

 should, accordingly, be corrected by subtracting E 2 -f- R v , where R v is 

 the resistance of the voltmeter. The whole correction for the watt- 

 meter is now E 2 (i/R w 4- i/R v ), which is to be subtracted from the 

 wattmeter reading. 



43. Use of an Ammeter with a Wattmeter. If an ammeter is 

 connected in circuit on the load side of a wattmeter, as the current 

 coil in Fig. 6, the ammeter reads the true load current. The watt- 

 meter reading, however, includes the watts loss in the ammeter a 

 small error which is neglected. 



If the ammeter is connected on the supply side of the wattmeter, 

 no error is introduced in the wattmeter reading ; the ammeter reading, 

 however, is too large,* since it includes the current in the potential 



* (43a). Ammeter Correction. The ammeter reading can be corrected 

 by subtracting (i/R v -\- i/R^W/I. The current which flows in the poten- 

 tial circuits of the voltmeter and wattmeter is E/R V -f- E/R V . This is in 

 phase with the electromotive force and not with the current, and must be 

 multiplied by the power factor of the load WIEl to get its component in 

 phase with the current. 



