  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
$\eps\to 0$.
