H. A. Rowland — Electrical Measurement. 445 



If we make R = we have 



b'rnL' = rR, 



M __r + R' + R, 



m r 



This method requires two simultaneous adjustments. M 

 must also be greater than m. As M and 1/ belong to the same 

 coil, we can consider this method as one for determining m in 

 terms the M and 1/ 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 24. 



M, ~ M' r' ' M' ~ r t (r' + R' + R") 

 This is a good method for comparing standards. We first 

 determine ■=£■ for each coil by one of the previous methods. 

 Then we can calculate -J and adjust the other resistances to 



balance. 



It is independent of the period of the current and suitable 

 for standards of equal as well as of different values, as the 

 mutual inductances can have any ratio to each other. 



For twisted wire coils r / = r 1 very nearly. See method 23 

 for the use of the resistance, A. 



Method 25. 



In fig. 6 remove the shunt W and self induction L. 



This method then depends upon the measurement of the 

 angular deflection when a self induction or a capacity is put in 

 the circuit of the small coil of the electrodynamometer and 

 comparing this with the deflection, when the circuit only con- 

 tains resistance. 



The resistance of the circuit, r, is supposed to be so great 

 compared with R that the current in the main circuit remains 

 practically unaltered during the change. 



There is also an error due to the mutual induction of the 

 electrodynamometer coils which vanishes when r is great. 



These formulas assume that the deflection is proportional to 



