ELECTRICITY METERS 459 



The lower end of the shaft is provided with a removable steel 

 pivot which rests in a sapphire or diamond jewel carried by a 

 spring support in the end of the jewel screw (see Fig. 267). This 

 screw can be turned back so that the disc D is clamped against 

 the magnets M\ the pivot is thus relieved of all strain during 

 transportation. 



The brake disc, 7), now made of aluminum, moves through the 

 fields of the permanent magnets, M. To change the retarding 

 torque and therefore the speed of the meter, the distance of the 

 poles of the magnets from the axis of the disc may be altered. 



In order to compensate at light loads for the effects of mechan- 

 ical friction, a field coil of fine wire, F', is connected in series with 

 the armature. A small permanent driving torque is thus ob- 

 tained. By adjusting the position of the coil with respect to the 

 armature this torque may be made such that the registration at 

 light loads is commercially correct; at the same time the load cur- 

 rent necessary to start the meter is much reduced. The effect 

 of the light-load coil at the higher loads is insignificant. 



To show that the number of revolutions during a given time is 

 practically proportional to the energy supplied to the load via 

 the meter, 



Let V = line voltage. 

 7 = line current. 



7 = current in potential circuit or armature. 

 R = resistance of potential circuit, 

 w = angular velocity of armature. 

 h = field due to drag magnets. 

 r = resistance to eddy currents in brake disc. 

 Ks = driving torque due to light-load coil. 

 KM = initial friction torque. 



K and k, with various subscripts, are constants or proportion- 

 ality factors. 



The flux through the armature due to the main coils, F, will be 

 proportional to 7 and that due to the starting or light-load coil, F f , 

 will be proportional to 7 a , so 



total flux through armature = k F I -f k F >I a . 

 The back e.m.f. in the armature circuit will be proportional to 

 the product of the flux and the angular velocity. 

 Back e.m.f. = fc(W -f WJw 



