ELECTRICITY METERS 461 



For meters as actually constructed 



watt-hours registered on dials = K h N. (5) 



This is fundamental and applies to all watt-hour meters. 

 The factor K h is called the watt-hour constant, and is the 

 number of watt-hours of energy necessary to cause one revolution 

 of the movable system. 



General Discussion of Essential Characteristics. A considera- 

 tion of the uses to which the watt-hour meter is put will show 

 that 'the instrument should possess certain characteristics. 



In order that the first cost and the expense of maintenance 

 may riot be too great, electricity meters must be simple in 

 design and must contain no parts which are subject to rapid 

 deterioration. 



It is desirable that the reading in kilowatt-hours be given 

 directly by the dials, especially in small meters; this avoids the 

 necessity for multiplying the dial readings by a constant. A 

 possible source of misunderstanding between the consumer and 

 the supply company is thus avoided. 



The meter should be protected by a case which can be sealed 

 and which is dust, water and insect proof; the arrangement 

 should be such that there is little likelihood that a short-cir- 

 cuit can occur during the removal and the replacement of the 

 meter cover. It is desirable that the electrical connections to 

 the meter be so made that it is not possible to tamper with the 

 instrument; in certain cases special devices are used for covering 

 all the connections from the service wires to the meter so that 

 it is impossible for the customer to draw current which does not 

 pass through the meter. 



Permanency of calibration is a prime requisite. On a large 

 system it is not practicable to test and adjust the majority of 

 the meters oftener than once a year. Consequently, they must 

 maintain their accuracy for at least this period. In the case of 

 large consumers where the amount of money involved is consider- 

 able the inspections are more frequent. 



To attain permanence of calibration the friction at the pivots 

 and commutator and the retarding torque of the magnetic brake 

 must remain constant. It is essential that the commutator and 

 brushes be capable of operating continuously for long periods 

 without undue increase of friction and without attention. To 



