  In this work we obtain Planck's blackbody spectrum from the thermal scalar
radiation contained in a resonant cavity of volume V in the context of
classical mechanics, which provides the classical zero-point electromagnetic
radiation in terms of a scale parameter that depends on geometric properties of
the enclosure and electrical magnitudes. The scale parameter of the classical
zero-point electromagnetic radiation is associated to experimental measurements
of Casimir forces, but we show that theoretically its value for known radiant
cavities that approach a blackbody has a numerical value of the order of
Planck's constant.
