THE MEASUREMENT OF CURRENT 81 



of two coils is 7 F / A/ -i0- the deflection of an electrodynamometer 



with coils of small cross-section thus proportioned, neglecting the 

 action of local fields, should be very nearly proportional to the 

 square of the current. Also, if the coils of a deflectional watt- 

 meter (page 306) be thus proportioned, the scale should be sen- 

 sibly uniform throughout the working range of the instrument. 



If the movable coil of a sensitive dynamometer, designed as 

 just suggested, is allowed to deflect and the angular movement 

 is read by the mirror and scale method, in general, when direct 

 currents are used the conditions are as indicated by Fig. 40. 



Fi<;. 40. Pertaining to effect of local field on a deflectional 

 < K-c trod ynamometer. 



If A is the equivalent area of the movable coil, H the st rength 

 of the local field, and 6 the deflection, 



T0i = KJrlu + I M AH sin (a + - 0i). 



If both currents are reversed the sign of the term involving H 



will bo changed, 



T0 2 = KJrI M - I M AH sin (a + ft - 2 ). 



In setting up the instrument is made as near and a as 

 noar <)0 as possible, then 



T0! = KJ F I. V + I> f AH(cos 00 

 and if tho coils are in serie 



T0i = A',/ 2 + I AH cos OL 



If tho instnimont is read by telescope and scale the angle of 

 o 



