10 Mr. A. Campbell on the Magnetic Fluxes in 



The power spent in the meter at full load = 12*8 watts. 



The power spent in turning the spindle (at 2'2 revolutions 

 per second) with full load was measured by the method 

 (1) described below, and was found to be 0*016 watts*. 

 Hence the efficiency of the meter as a motor = 0*125 per cent. 



Kelvin Ampere-hour Meter. — "Range to 600 amperes. In 

 this a thin iron core, kept highly magnetized by a shunt- 

 current, is drawn down into a solenoid which carries the main 

 current. The solenoid had 6 turns (i. e. 3600 ampere-turns 

 at full load), and was about 16 centim. long. At full load 

 the flux density at the lower end of the solenoid was over 

 250. With the shunt-current alone the total flux through the 

 moving coil and core was about 330. 



Eliliu Thomson Watt-hour Meter. — Range to 50 amperes 

 at 100 volts. This meter consists of a small ironiess motor, in 

 which the series-current goes through the field- magnet coils 

 and the shunt-current through the armature. On the arma- 

 ture spindle is a brake disk of copper (13*5 centim. diameter), 

 wbich passes between the narrow air-gaps of three permanent 

 magnets of the shape shown in fig. 4. These magnets are 

 often of different strengths, being chosen to give the proper 

 brake-force for each individual meter. Their polar faces are 

 about 7*5 sq. centim. The mean B in the air-gaps was about 

 700. By the method of placing the magnets the greater part 

 of the flux acts on the brake disk. By slipping a search-coil 

 along one of the magnets it was found that the total flux at 

 a was about 15000, making B about 7000. Of this flux 

 nearly one half remains in the steel as far as the section at b. 



Fig. 4. 



Driving Flux. — Without the shunt-current the full load 

 current gives mean B == 130 along the axis of the series-coils. 

 The shunt-current at 100 volts gives a field at right angles 

 1o this in which B = 10. The shunt-current also passes 

 through a " compounding " coil fixed coaxially in one of the 



* After the author had measured the motor-efficiencies of several 

 meters, M?\ Sidney Evershed somewhat anticipated him by announcing 

 (Institution of Electrical Engineers, May 12th, 1898) one or two similar 

 results, making no mention, however, of the method by which the results 

 were obtained. 



