ELECTRICAL MEASUREMENT BY ALTERNATING CURRENTS 309 

 To find the ratio for any other coil independent of the induction coil, 



TJ 



we can first find ^ as above. Then add L to the same circuit and we 

 M 



L 4- L' 

 can find ^, Whence we can get L. This seems a convenient 



jj 



method if it is sensitive enough, as the value of -jj, should be accurately 



jd 



known for the inductance standard. 



Method 19. 

 'l-M*} = S- [RR t -R"Rl 



L' _R' + RL'l-M*l ,,\_K + R. R'R^-R'R.jl , , 



~ ~ ~~~ ~~ * 



M~ r r* \M 



This is useful in obtaining the constants of an induction standard. 

 For twisted wires L'l M 2 should be nearly 0, depending, as it does, 



on the magnetic leakage between the coils, -^.is often known suffi- 



ciently nearly for substitution in the right hand member. It can, 

 however, be found by reversing the inductance standard. 



Method 20. 

 R'R tl - R'R, = 

 W R L 



L' any value. 



In case of a standard inductance, M and L are known, especially 

 when the wires are twisted. 



The method can then be used for determining any other inductance, 

 L', and is very convenient for the purpose. 



R n and R t + R tl are first calculated from the inductance standard. 

 The Wheatstone bridge is then adjusted and W varied until a balance 

 is obtained. This balance is independent of the current period, as also 

 in the next two methods. 



Method 21. 

 R'R tl - R"R, = 



I _R' + R, L' _(K + Rp. L' _R + R ll ^M 

 M -- ^^ ; Tt~ rR, T = ~^T~ 



This is Niven's method adapted to alternating currents. See re- 

 marks to method 20. 



