12 Mr. A. Campbell on the Magnetic Fluxes in 



switched on, the tangential force /necessary to hold at rest 



the end of this arm was measured by the extension of a spiral 



spring which had been calibrated by known weights. 



Then 



n 

 Power (in watts) = 10 -7 ./. 27rr . — , 



where 



r = length of arm, 



- = number of revolutions per second when the spindle 

 is free to move. 



(2) In the second method the shunt-circuit was disconnected 

 and joined directly to a sensitive galvanometer with a resist- 

 ance of 12,000 ohms in circuit. A measured current was 

 sent through the series-coils, and the spindle was turned at 

 about the full rate. Thus the meter acted as a dynamo and 

 gave a deflexion on the galvanometer. By watching the gal- 

 vanometer-scale it was not hard to keep this deflexion steady, 

 and from the known calibration of the galvanometer the 

 voltage given by the armature was obtained. The total 

 E.M.F. multiplied by the normal shunt-current (0*0493 amp.) 

 gave the driving-power. 



Method (1) gave for the driving-power at full load 0*020 

 watt, whilst method (2) gave 0*021 watt. Hence the efficiency 

 of the meter as a motor is only 0*095 per cent 



Hoohham Alternating- Current Watt-hour Meter. — Eange to 

 10 amperes at 100 volts. In this meter a solid iron core 

 forming an almost closed magnetic circuit is magnetized by 

 a shunt-coil, which latter, by reason of its large inductance, 

 carries a current which lags behind the main current by 

 50° to 60°. A smaller U-shaped piece magnetized by the 

 main current has its poles close to the upper pole of the 

 larger iron core, and one of the poles carries a copper screen. 

 A small aluminium disk (8*8 centim. in diameter), partly 

 between the poles, is turned by the rotary magnetic flux 

 thus produced. The brake force acts on the same disk, and 

 is due to a tall permanent magnet about 20 centiin. high 

 (with a narrow air-gap) , as shown in fig. 5. 



Brake Flux. — The maximum flux in this magnet was 

 found to be near P, and had the value 19,820, corresponding 

 to B = 8800. Of this only 4400 lines ultimately cut the 

 disk, much being lost by cross leakage from Q and S to the 

 opposite limb. Thus less than J of the maximum flux is 

 made use of. The mean B between the poles was found to 

 be 650. 



