Meters and other Electrical Instruments. 17 



take that a volume of about 61 c.c. was raised in temperature 



by 0°'87 C. 

 Now 



rower spent = 4*2 watts, 



where <r = sp. heat of the iron, 



V = volume „ „ 



D = density „ ,, 



^T= temperature-rise (corrected), 



t = time in seconds; 

 therefore from the results above, 



Power = 1*58 watts. 



Since the hysteresis-loss is about 0*10 watt, it will be seen 

 that the eddy-current loss = 1*48 watts. 



The power lost by eddy-currents in the disk was similarly 

 measured, and was found to be about 0*02 watt. 



In the shunt-coil the C' 2 R loss =0*57 watt ; and hence the 

 total copper and iron losses =2*17 watts. 



Direct Measurement of Total Power. — The total power 

 given to the meter was measured by the three-voltmeter 

 method, in which was used a reflecting electrostatic voltmeter 

 accurate to about 1 in 1000 at all the points of the scale 

 required. The result obtained was 2*06 watts, which agrees 

 fairly well with the total watts shown by the other methods. 



Driving- Power. — The driving-power at full load was also 

 measured by the method of the spring-balance described 

 above ; it was found to be 0*00073 watts. Taking account 

 of the series-coil (whose resistance was 0*009 ohm) the total 

 power spent at full load is over 3 watts ; and hence 



Motor efficiency = 0*024 per cent. 



Scheefer Alternating- Current Watt-hour Meter. — Range to 

 10 amperes at 200 volts. In this meter a pile of E-shaped 

 iron stampings has the series-coil on one outer limb and the 

 shunt-coil on the other. An aluminium cylinder is turned 

 by the rotary field thus produced, and on the same spindle is 

 a brake disk with a single magnet exactly like those in the 

 Elihu Thomson meter. 



The mean B between the poles of this magnet was about 

 480. With the shunt-current alone the -v/mean B 2 between 

 the shunt-pole and the driving cylinder was 570. The total 

 power spent with the shunt alone on is 11*5 watts, of which 

 7*1 watts are due to C 2 R loss. 



Shallenberger Alternating- Current Ampere-hour Meter. — ■ 

 Range to 20 amperes. A series-coil (of about 50 turns) and 



Phil. Mag, S. 5. Vol. 47. No. 284. Jan. 1899. C 



