  We trace the historical appearance of Planck's constant in physics, and we
note that initially the constant did not appear in connection with quanta.
Furthermore, we emphasize that Planck's constant can appear in both classical
and quantum theories. In both theories, Planck's constant sets the scale of
atomic phenomena. However, the roles played in the foundations of the theories
are sharply different. In quantum theory, Planck's constant is crucial to the
structure of the theory. On the other hand, in classical electrodynamics,
Planck's constant is optional, since it appears only as the scale factor for
the (homogeneous) source-free contribution to the general solution of Maxwell's
equations. Since classical electrodynamics can be solved while taking the
homogenous source-free contribution in the solution as zero or non-zero, there
are naturally two different theories of classical electrodynamics, one in which
Planck's constant is taken as zero and one where it is taken as non-zero. The
textbooks of classical electromagnetism present only the version in which
Planck's constant is taken to vanish.
