50 ELEMENTS OF ELECTRICAL ENGINEERING. 



of watt-hour meter is exemplified by the Thomson meter which 

 is described in Art. 92 of the first volume of this text. This 

 meter can be used on either direct-current circuits or alternating- 

 current circuits, but the induction meter is suitable for alternating- 

 current circuits, only. The induction meter is generally used on 

 alternating-current circuits because of its simplicity and cheapness. 



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Fig. 40. 



In the induction watt-hour meter a thin disk of aluminum or 

 copper is mounted on a delicately pivoted spindle and driven by 

 an electromagnetic device at a speed proportional to the power 

 delivered to the receiving circuit. The total number of revolu- 

 tions of the spindle is therefore proportional to the work delivered 

 to the receiving circuit, and this is indicated directly in watt-hours 

 by dials which are driven by the rotating spindle. 



The essential features of the electromagnetic device for driving 



