  The article discusses a theory that allows to describe losses in conductors
taking into account the skin effect and the proximity effect. The developed
theory is applied to inductance coils. In contrast to the classical Butterworth
theory the condition of the strong skin effect, which is frequent in practice,
is introduced from the very beginning. This approach makes possible to obtain
extremely simple formulas, which show that Q-factor of a coil is determined
mainly by the diameter of its winding.
