18 Magnetic Fluxes in Meters and other Electrical Instruments. 



a small short-circuited secondary coil, with their axes at an 

 angle of about 45°, produce a rotary field by which is turned 

 a small disk with a soft iron rim. The brake-force is obtained 

 by air-friction on four aluminium vanes. 



By the search-coil and thermopile method it was found that 

 inside the series-coil 



VmeanB 2 =100. 



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

 was 0'025 ohm ; hence the power spent in it =10*0 watts. 



The power spent in the copper stampings which form the 

 secondary coil was found by measuring their rise of tem- 

 perature with a small copper-iron junction. This rise (at 

 full load) was found to be about o, 33 C. per minute, the 

 cooling being negligible. The volume of copper was about 

 21 '4 c.c. ; whence the power spent = 0'40 watt. 



The driving-power was found to be 0*0069 watt ; therefore 

 the motor efficiency =0'066 per cent. 



Current in Secondary Coil. — The current in the short- 

 circuited secondary coil could not be measured directly. 

 Calculating from the dimensions of the coil, however, the 

 resistance was foimd to be 8*5 x 10 -6 ohm. From this and 

 the value of the power (0*40 watt) we find that the secondary 

 current attains the extraordinary value of 220 amperes. 



For the sake of comparison some of the above results are 

 collected in Table IV. 



Table IV. 



Name. 



Driving B. 



Brake B. 



Power 



spent. 



Motor 

 Efficiency. 



Elihu Thomson 



130 



(not 

 measured) 



50 

 100 



700 



1020 



650 



watts. 

 21-4 



12-8 



3-2 



10-4 



per cent. 

 0-095 



0-125 



0-024 



0066 



Hookham (direct curr.) ... 

 Hookham (alternat. curr.) 

 Shallenberger 





In conclusion, it will be noticed that in motor meters the 

 driving-flux density is of the order 100 and the brake B from 

 500 to 1000 ; also that the motor efficiencies are all very 

 small, particularly in the case of the alternating-current 

 meters. In all of them the greater part of the power taken 

 is spent in heating conductors (either by eddy-currents 

 or otherwise). If a small fraction of this wasted energy 

 could be employed to overcome with certainty the friction at 

 the lowest loads, a great advantage would be gained thereby. 



June 7th, 



