We study the asymptotic behaviour of the inductance coefficient for a thin
toroidal inductor whose thickness depends on a small parameter \eps>0. We
give an explicit form of the singular part of the corresponding potential
u\ue which allows to construct the limit potential u (as \eps\to 0) and
an approximation of the inductance coefficient L\ue. We establish some
estimates of the deviation u\ue-u and of the error of approximation of the
inductance. We show that L\ue behaves asymptotically as \ln\eps, when
\eps\to 0