Meters and other Electrical Instruments. 



11 



series-coils for the purpose of overcoming friction at the lower 

 loads. The B due to this starting coil alone is about 3. 

 There is a small stray field from the series-coils perceptible at 

 the brake disk, but as it is less than the ^Jq part of the field 

 due to the permanent magnets its effect may be neglected. 



Effect of the Earth's Field. — Since the driving flux density 

 at full load is only 130, it is clear that at the lower loads the 

 rate of the meter may be considerably affected by the earth's 

 field. To test this point the meter was levelled up on a turn- 

 table (as in the case of the electrodynamometer already 

 described), and a constant load of 4*625 amperes at 100 volts 

 was kept on it. The load was measured by a Kelvin balance 

 and a reflecting multicellular voltmeter. The rate of the 

 meter, i. e. spindle revolutions per watt-second, was then 

 determined 



(A) with alternating current : 



(B) with direct current, the earth's field helping the driving- 

 field ; 



(C'i the same, with the earth's field opposing the diiving 



field. 

 Table I1L shows the results of the tests. 



Table III. 



Position of Meter. 



Current. 



Rate. 



(A) Facing East. 



, (B) „ West. 

 (C) „ East, 



Alternating at 80^ 

 per second. 



Direct. 



Direct, 



0-C002125 



0-0002135 

 00002212 



It will be seen from (B) and (C) that by turning the meter 

 round through 180° its rate at ^ l° a ^ ^ s altered by 3*6 per 

 cent., which is exactly what might have been predicted from 

 the value of B given above. The rate with alternating current 

 does not lie between the rates (B) and (C) as it ought, but is 

 about 5 per cent, slower than (B). 



Power Spent in Meter. — The resistance of the series-coil 

 = 0*0066 ohm, and that of the shunt-coil = 2030 ohms ; 

 hence the power spent in heating the coils =16*5 + 4-9 = 21*1 

 watts. 



The power spent in actually driving the meter (at full load) 

 was measured by two methods as follows : — 



(1) An arm of about 10 centim. long was attached to the 

 spindle of the meter (at right angles to it). With full load 



