458 



ELECTRICAL MEASUREMENTS 



motor is proportional to VI and the retarding torque due to the 

 brake is proportional to the angular velocity of the disc. Broadly 

 speaking, therefore, the energy supplied to the circuit during a 

 given time will be proportional to the number of revolutions of 

 the armature during that time; in other words, to the reading 

 on the dials of the counter. 



Referring to Fig. 265, the main current passes through two 

 similar field coils of low resistance F. In a two-wire meter these 

 coils are connected in series. The potential circuit, which con- 

 tains the armature A , is connected on the line side of the main 



FIG. 265. Thomson watt-hour meter for direct currents. 



coils. The armature (now made in a spherical form to reduce 

 the weight) is carried by an upright spindle which at its upper 

 end gears by means of a worm into the very light train of wheels 

 which moves the pointers of the counter over the dials. The 

 current is carried to the armature through silver-tipped brushes, 

 which rest with a very slight pressure on a silver commutator of 

 small diameter. The brushes are adjusted before the meter 

 leaves the factory and should be very carefully handled. 



In the early designs of this meter, the series resistance, R, was 

 wound non-inductively on cards and carried in an envelope at 

 the back of the instrument. It is now combined with the light- 

 load coil, F', mentioned below. In a 110- volt instrument, the 

 total resistance of the potential circuit is about 2,500 ohms. 



