We discuss the vacuum energy of the electromagnetic ¯eld interacting with a
spherical plasma shell together with a model for the classical motion of the shell. We discuss
the ultraviolet divergences in terms of the heat kernel coe±cients. Using these, we carry out
the renormalization by rede¯ning the parameters of the classical model. It turns out that this is
possible and that the resulting model has a vacuum energy which changes sign in dependence on
the parameters of the plasma shell. In the limit of the plasma shell becoming an ideal conductor
the vacuum energy found by Boyer in 1968 is reproduced
