158 ELECTRICAL MEASUREMENTS 



the wires are wound throughout their length side by side and in 

 layers. Any residual magnetic effect, if the instrument be of the 

 Kelvin type, may be annulled by a small coil placed outside the 

 case of the instrument and connected in series with the weaker 

 coil in such a manner that its effect is additive; this adjusting 

 coil is mounted so that its position may be altered by sliding it 

 along a rod which is coaxial with the main coil. 



The simplest method of using the instrument is shown in 

 Fig. 84. 



On the diagram R Gx and R Ga are the resistances of the two gal- 

 vanometer coils; L x and L 8 the total lead resistances; R x and R, 

 the resistances unplugged in the boxes. To avoid leakage and 

 capacity effects, the positions of the boxes R x and R 8 should be 

 such that the potential difference between the two galvanometer 



FIG. 84. Simple method of using differential galvanometer. 



coils is a minimum. The resistance of a should be low. High 

 insulation of the leads, etc., is necessary. 



First, the adjustment of the instrument must be tested. To 

 do this the coils are connected in series and opposed magnetically; 

 the maximum working current is then sent through them. No 

 deflection should be observable. Perfect adjustment is obtained 

 by adjusting the auxiliary coil. 



With the connection shown in Fig. 84, after having unplugged 

 a suitable resistance in R x , the value of R 8 is adjusted until the 

 galvanometer stands at zero; then the currents in the two coils are 

 equal, and 



X = R Gx + R X + L X + X 

 S " R 



or Y Q Gx "" * * 



A "= & -p - i p I r 



KG, + R s + L a 

 The galvanometer and lead resistances must be known. 



