310 HEXEY A. EOWLAXD 



Methods 20 and 21 are specially useful when one wishes to set up an 

 apparatus for measuring self induction, as the resistances R', R", 

 R t , R lt can be adjusted once for all in case of a given induction standard 

 and only W or r need be varied afterwards. 



Method 22. 

 L '1 = KA. M =R R"- ^ = R" (i 



This is Carey Foster's method adapted to alternating currents and 

 changed by making R" finite instead of zero. 



The ratio of R' -f- R, to R t is computed from the known value of 

 the induction standard. R" is then adjusted and C" obtained. In 

 general the adjustment can be obtained by changing R t and R". The 

 adjustment is independent of the current period. 



Method 23. 



"rJvA^r+s+n, 



m 

 If we make R = we have 



tfmL' = rR t 



M^r+R' + R, 



m ~ r 



This method requires two simultaneous adjustments. M must also 

 be greater than m. As M and L' belong to the same coil, we can con- 

 sider this method as one for determining m in terms of the M and L' of 

 some standard coil. 



The resistance, A, can be varied to test for, or even correct, the error 

 due to electrostatic action between the wires of the induction standard. 



Method 2.L 



M t M'r" M'~r,( 

 This is a good method for comparing standards. We first determine 



-^ for each coil by one of the previous methods. Then we can calcu- 

 late ^ and adjust the other resistances to balance. 



It is independent of the period of the current and suitable for stand- 



