274 On the Inductance of Two Parallel Wires. 



If L be the self-induction of the system, uhp is the 

 imaginary part of 



4pa 



or of 



4/?a 



~8 



to the same order. 



The first term of this expression leads to the results already 

 given. The second term, writing 



„— K./(log*a-jgj) 



1 + pi = 2 practically, 



becomes 



4^. Vc/ * aJ ° 



l0 ^)"S 



I£ R be the real part of 



a jjlJ 



LOff 



where 



and the argument of the Bessel functions is in all cases 



ka = ociW = 2a ^y 7 ^ . ^ as before. . (106) 



Then the self-induction is 



T . , c , 4a ber#ber'# + bei x bei' # , ^ ,+ A n \ 



This formula may be used even when c = 3a, to obtain a 

 result not in error by more than one per cent. For the 

 application of this result, the functions analogous to ber <r, 

 and bei# derived from the Bessel function of order 1, require 

 tabulation. 



