  Stern-Gerlach experiment is a paradigm of measurement theory in quantum
mechanics. Notwithstanding several analysis given in literature, no clear
understanding of the apparent collapse has been given so far. Indeed, one can
imagine a Stern-Gerlach device where all environmental effects are removed and
ask the question on how the measurement goes on. In this letter we will prove
that a the Stern-Gerlach device behaves as a true measurement apparatus, as
expected by the Copenaghen interpretation, by the Ehrenfest theorem. In this
way we recover, by other means, a limit on the Stern-Gerlach device, given by
Bohm, based on scrambling of phases due to a large oscillation frequency.
