  We have studied theoretically the Purcell factor which characterizes a change
in the emission rate of an electric or magnetic dipole embedded in the center
of a spherical cavity. The main attention is paid to the analysis of cavities
with radii small compared to the wavelength. It is shown that the Purcell
factor in small metallic cavities varies in a wide range depending on the ratio
of the cavity size to the skin depth.
