H. A. Rowland — Electrical Measurement. 443 



By this method the self induction of the mutual induction 

 coil is eliminated. But it is difficult to apply, as two resistances 

 must be adjusted and the adjustment will only hold while the 

 current period remains constant. The same remarks apply to 

 B and C following. 



(B) E = oo 



om1 ^ - w"'+r,+r„ 



U_ _ W , "[R , + R / + R ,, + RJt(R , + R / )(R ,, + R") 



M ~ R;,W 



(C) W = oo 



L^ _ R(R' + R, + R" + R„) + (R' + R,)(R" + R„ ) 

 M " RR y/ 



Method 18. 



R / R"-RR // =0 



fr R" 'r' + R" 



■Mil — ^~ 



M' ' R" . ' W 



1/ and M 7 belong to the same coil. By adjusting the Wheat- 

 stone bridge first, W can then be afterwards adjusted. 



To find the ratio for any other coil independent of the in- 

 duction coil, we can first find ^ as above,. Then add L to the 



M' 



T , T / 



same circuit and we can find — — — . Whence we can get L. 

 This seems a convenient method if it is sensitive enough, as 



the value of — , should be accuratelv known for the inductance 



M' 



standard. 



Method 19. 



b\Vl-W) = ~ [R'R^-R'TIJ 



L' R' + R, L7-MV £ \_ R+R, R'R„-R'R /; \ 



M~ r r 2 \M + / r rU n \M / 



This is useful in obtaining the constants of an induction 

 standard. For twisted wires Ul — M 2 should be nearly 0, de- 

 pending, as it does, on the magnetic leakage between the coils. 



— is often known sufficiently nearly for substitution in the 



right hand member. It can, hovever, be found by reversing 

 the inductance standard. 



