CHAPTER IX 

 ELECTRICITY METERS 



Watt-hour Meter. In connection with the supply of elec- 

 trical energy for lighting and power, it is necessary to have some 

 form of integrating meter; that is, a meter which will give, 

 not the rate at which energy is supplied to the circuit, but the 



total amount of energy supplied during a given time, as for in- 



rt* 

 vidt; v and i 



are the instantaneous values of the voltage and current; the 

 time during which the energy is supplied is Z 2 t\. It is custom- 

 ary to express this time in hours. The energy is then stated in 

 watt-hours, or, more frequently, in kilowatt-hours. 



The necessity for accurately measuring electrical energy is 

 apparent from the fact that in the United States, alone, the 

 charges for the electrical energy furnished for light and power 

 were, for the year 1916, approximately $450,000,000. 



The essentials of the watt-hour meter will be better appreciated 

 if one approved form of the instrument be described. The 

 Thomson watt-hour meter for direct currents will be selected, 

 for this was the first successful commutating meter. It was 

 placed on the market in the latter part of 1889, has passed 

 through the usual processes of development, and is still regarded 

 as one of the best of its class. 



The instrument consists of a small motor which is provided 

 with a magnetic brake. The motor drives a counter whose indi- 

 cations on a system of dials are proportional to the total number 

 of revolutions which have been executed by the armature. 



No iron is used in either the field or the armature of the motor, 

 therefore all magnetic effects are directly proportional to the 

 currents. 



The fields of the motor are placed in the main circuit in series 

 with the load. The armature, in series with a suitable resistance, 

 is connected across the supply mains. The driving torque of the 



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