Variable Mutual Inductances. 
169 
which is rather laborious, the following approximate 
values were found, when A = 20, a=10, b = 5 cm., and 
n\ n 2 = 100,000: — 
?• 
M. 
cm. 
millihenries. 

9-3 
5 
9-7 
10 
9-9 
15 
6-7 
It is interesting to notice that the maximum value of M 
does not occur when the axes of the coils are coincident 
(<7 = 0). To avoid the labour of the calculation by quadra- 
tures, some rough experiments were made with coils of finite 
section (about 0*6 x0*6 sq. cm.). From one set of these the 
series of curves shown in fio-. 13 was obtained. The coils 
Fiff. 13. 
/\ = 10 , a. - 5 <**~ 
/lt / <fc z .= 10 OOOO 
were of mean radii A =10, a = 5 cm. respectively, n^ n 2 
being 100,000, and q and b were given various values. It 
will be seen that for b = 2'6 the maximum M does not occur 
at the symmetrical position (just as in the case calculated 
above). For equal coils, however, the coaxial position gives 
