.1 MAGNETIC CL'RVE TRACI-.R 



27 



current in is varied slowly enough, the lime lag in the secondary 

 circuit will be negligible and we shall have for the instantaneous 

 current in the fluxmeter: 



/„ = 



Rs + Ra + 



R, 



Now the e.m.f. in the search coil is 



e = - /lA 



dt 



where A is the cross sectional area of the specimen and N is the number 

 of turns in the search coil. Then 



AN 



In = 



d{B - //; 

 dt 



and therefore 



Q - io 

 Jo 



But by Eq. (1) 



therefore 



.dl = 



R, + Rjl-h^^ 



- AN 



Rs + Rg 



Q = - K8 





A{B - H) = 



\r. 



Kb\Rs + RA^ +^ 



AN 



(2) 



where 



A' = 





ro being the total secondary resistance when K was determined. This 

 equation, then, gives B — II for any given deflection 5, in terms of 

 known constants. For any fluxmeter, K is determined once for all by 

 passing the current im through a mutual inductance and measuring the 

 deflection Bm on a photographic record. The other constants are 

 changed in a calculable way when the number of turns in the search 

 coil, the resistance settings, and the cross-sectional area of the sample 

 are changed. 



Sources of Error 



Since it is the voltage applied to the magnetizing coil F which is 

 proportional to the angle through which the drum has rotated, there 



