The mutual inductance of two coaxial circles of radius and with centers at a distance apart is given in cgs units by
where is the speed of light, and in (19.29.11),
or, by (19.21.3),
A simpler form of the result is
References for other inductance problems solvable in terms of elliptic integrals are given in Grover (1946, pp. 8 and 283).