416 ELECTRICAL MEASUREMENTS 



coils and again measure the net inductance. One measurement 

 gives the sum, the other the difference of the mutual and self- 

 inductance effects. If the two net inductances are L' and L", 

 and the self-inductances of the coils are L\ and L 2 , respectively, 



L' = L, + L z + 2m 

 L" = L l +L 2 - 2m. 



Consequently the mutual inductance is given by 



r / T ft 



m = -'' (48) 



Maxwell Method of Comparing Mutual Inductances. The 

 connections necessary for comparing two mutual inductances 

 by Maxwell's method are shown in Fig. 242 where it is indicated 



o> 



FIG. 242. Mesh diagram for Maxwell method of comparing mutual 

 inductances. 



that alternating currents replace the variable currents assumed 

 in the original demonstration.* The two mutual inductances 

 are m x and m P ; Z x and Z P are the impedances of the respective 

 meshes omitting the detector; L' is a variable self -inductance 

 which can be used in series with either m x or m P . Its value need 

 not be known. 



The mesh equations are 



XZ X YZ G jum x I B = 

 XZ P + Y(Z P -f Z ] jum P I B - 

 from which the detector current is 



Z x m P Z P m x 



Treatise on Electricity and Magnetism," third edition, Art. 775. 



