ALTERNATING-CURRENT INSTRUMENTS 



57 



and this is the difference of potential which exists between the 

 fixed and moving coils. This is dangerous from the insulation 

 standpoint, and electrostatic forces existing between the fixed 

 and the moving coils may cause an error in the instrument read- 

 ing. (The wattmeter is also briefly described in Vol. I, Chap. VII.) 



Load 



FIG. 53. Incorrect method for connecting a wattmeter. 



33. Wattmeter Connections. In Fig. 54 (a), wattmeter W is 

 shown measuring the power taken by a certain load. In order 

 to measure this power correctly, the wattmeter current-coil should 

 carry the load current, and the wattmeter voltage-coil, in series 

 with its resistance, should be connected directly across the load. 



Load 



Instrument measures power <-<>n- 

 sum< A n current <-<>il. 



(*) 



In>t ruiiM-nt measures power consumed 

 by its own potential circuit. 



. 54. Methods for connecting a watliii. 



The current in the wattmeter current-coil i> the same as the 

 load current, but the wattmeter potential-circuit is not coin, 

 directly across the load, hut is measurim: a potential in excess of 

 the load potential ly the amount of the impedance drop in the 

 wattmeter ciinvnt coil. Therefore, the wattmeter reads too 

 high l>v the amount of po , -unied in its own current-coil. 



Under these condition- the true power 



/' - P' - /'-'A' 



