456 ELECTRICAL MEASUREMENTS 



rise. Consequently, unless there is some special compensation 

 for temperature effects, this form of wattmeter has a large tem- 

 perature coefficient. In the Westinghouse instrument shown on 

 page 452 the compensation is made, as in the ammeter, by ad- 

 justing the temperature coefficient of the circuit containing R z . 



For the discussion of the induction watt-hour meter, see the 

 chapter on "Electricity Meters," page 457. 



The advantages claimed for the induction type of instrument 

 for switchboard work, are : 



1. Extremely long scales, due to the fact that the movable 

 element can turn' through nearly 360. 



2. Compactness; this reduces the size of the switchboard panel. 



3. Errors due to external fields of the fundamental frequency 

 are small. 



4. Robust construction, which facilitates repairs. 



5. The ratio of torque to weight of moving element is large. 

 There are also certain disadvantages, some of which may be 



overcome by careful design. 



1. The indications are affected by changes of frequency and 

 wave form; these errors are very important in induction watt- 

 hour meters, especially at low power factors. 



2. There are temperature errors caused by changes in the 

 resistance of the armature due both to changes of room tem- 

 perature and to self-heating in the instrument. 



3. The torque always increases with the load on the instrument, 

 so in cases of sudden overload the pointer may be thrown violently 

 against the stop and either bent or displaced. 



4. Alternating currents must be used in checking the instru- 

 ments, this is sometimes inconvenient when dealing with 

 portable instruments. 



References 



1. "Electrodynamic Rotation Produced by Alternate Currents," G. 

 FERRARIS, The Electrician, vol. 36, 1895-96, p. 281. (Paper was read March 

 18, 1888.) 



2. "A New Form of Induction Ammeter or Voltmeter," PAUL MAO 

 GAHAN, Electric Journal, vol. 4, 1907, p. 113. 



3. "Induction Type Indicating Instruments," P. MACGAHAN, Trans 

 American Institute Electrical Engineers, vol. 31, part II, 1912, p. 1565. 



4. "Electricity Meters," H. G. SOLOMON, 1906, Charles Griffin & Co., 

 London. 



