ALTERNATING-CURRENT l.\*Tltr.MKNTS 



61 



of a polyphase instrument. With two individual instrui: 

 it is a simple matter to apply scale corrections. 



Wnttineter Calibration. A dynamometer-wattmeter is ordi- 

 narily calibrated with direct current, the connections for 

 calibration being shown in Fig. 58. The voltage across the 

 tial-circuit is measured with a standard directs unvnt volt- 

 meter. The current is accurately measured by means of a 

 potentiometer, although a standardized direct-current ammeter 

 is often sufficiently accurate. Both the current and the po- 

 tential are reversed at each reading so as to eliminate the efiVet 

 of the earth's field or of any stray field. The true power in watts 

 is given by the product of the current and the voltage, as direct 

 current is used. 



IRON-VANE INSTRUMENTS 



34. Voltmeters. In Vol. I, Chap. VII, it was pointed out that 

 inst rument s depending upon the solenoid act ion of an iron plunger 

 were not satisfactory as ammei 

 By the use of light iron vanes, jew- 

 elled bearing -at isfactory types 

 of commercial alternat ing-current 

 instruments, based on the principle of 

 magnetized iron, have been developed. 



< MM- -uch type of instrument, manu- 

 factured by the West on Klectrical 

 Instrument Co.. is shown in Fin. .V.i. 



A small strip of soft iron. .17. bent 

 into cylindrical form. i< mounted 

 ily on a >pindle which is free to 

 turn. Another similar -trip. /''. which 

 i> mon Of I' -haped. and with 



a larger radius than M . i- fixed inside 

 lindrical coil. The cylindrical 

 coil is wound with line wire and 

 a hiirh refllStanoe. When conn. -de. 1 | e line, t! 



throimh the instrument is sub-f ant ially proportional ' 

 cuit voltage. When current Hows thmuuh thlfl exciiii.- 



both iron vanes become magnetiied The r, |et of tlu 



trips will always have the same magnetic polarity, and the 



59. Weeton iron-vane 







connected in series with 



