  A classical model is presented for persistent currents in superconductors.
Their existence is argued to be warranted because their decay would violate the
second law of thermodynamics. This conclusion is achieved by analyzing
comparatively Ohm's law and the Joule effect in normal metals and
superconducting materials. Whereas Ohm's law applies in identical terms in both
cases, the Joule effect is shown to cause the temperature of a superconducting
sample to \textit{decrease}. An experiment is proposed to check the validity of
this work in superconductors of both types I and II.
