130 COEFFICIENTS OF INDUCTION. 



764. We shall pass from the case of two circles to that of a 

 coil and a circle, or of two coils with the same axis, by the method 

 of (743). 



If a and a' are the mean radii of the two coils, and x the distance 

 of their median planes, 2C and 26, ic and 2$', the dimensions of the 

 two channels ; then if M is the coefficient relative to the two mean 

 circles, 



If M is expressed by formula (19), we find, taking into account 

 known ratios, 



-8F E F 3E i,_ _. 



= --> (E ~ F) ' 



relative to the complete integrals, 

 v I E 



} 



*<i-*>jr 



765. The calculation will be more easily made by means of the 

 tables, if we use the following method due to Lord Rayleigh.* 



Let n 1 and n! be the numbers of windings of the coils. 

 a and a' their mean radii. 

 x the distance of the centres. 

 2^, 2c, 2b', and 2c' the dimensions of the channels. 



* MAXWELL. Electricity and Magnetism. 2nd Edition, Vol. n., p. 326. 



