certain Bessel Integrals. 339 
we have a solenoid of length 2h and radius a, and a circle 
of the same radius at a distance c from the central section of 
-h--> 
t~e~, 
■X 
the coil, we obtain from (16) 
- L 
= inVn[ 1- j -| r^' -." •"} JiV)f ]• (24) 
Substituting for these integrals from (7) and (17) according 
as /i+c and h — c are large or small compared with a, we can 
obtain series giving the induction through any section of a 
solenoid, whether it is a long or a short coil. 
§ 3. Two Coaxial Solenoids of Equal Length. 
<- Jv 
-> 
m 
s 
Suppose we have two single-layer coils of equal length 
2h and of radii a and 6, placed as in the figure ; let n x and 
