ELECTRICAL MEASUREMENTS. 291 



by equation (40) in which irZr 2 - may be written for A, giving 



T= irZ^IH' (i) 



At the same time the deflection <f> of the needle of the galvanom- 

 eter is observed so that, according to equation (36^), we have 



rH' 

 I ~ tan 6 (ii) 



The mean radius r and number of turns of wire Z in the coil 

 being known, and T and < being observed, these two equa- 

 tions determine the values of both / and H' . 



