  In this paper we calculate charge fluctuations of a Schwarzschild black-hole
of mass $M$ confined within a perfectly reflecting cavity of radius R in
thermal equilibrium with various species of radiation and fermions . Charge
conservation is constrained by a Lagrange multiplier (the chemical potential).
Black hole charge fluctuations are expected owing to continuous absorption and
emission of particles by the black hole. For black holes much more massive than
$10^{16} g$ , these fluctuations are exponentially suppressed. For black holes
lighter than this, the Schwarzschild black hole is unstable under charge
fluctuations for almost every possible size of the confining vessel. The
stability regime and the fluctuations are calculated through the second
derivative of the entropy with respect to the charge. The expression obtained
contains many puzzling terms besides the expected thermodynamical fluctuations:
terms corresponding to instabilities that do not depend on the specific value
of charge of the charge carriers and one of them depends on Newton's constant
instead. One of the contributions to the charge fluctuations $\hbar/4\pi$ does
not depend neither on number of species, nor on the the specific charge or even
the size of the confining vessel. As a matter of fact, this term emerges from
the second derivative of the black hole entropy alone, which means that it
corresponds to a genuine quantum mechanical property of the black hole itself.
Such a contribution would cause the event horizon to recede from $2M$ to
$2M-T_{BH}$ or equivalently, by $(4\pi)^{-1}$ of the black hole' s Compton wave
length. Similarly, a Cauchy horizon emerges at the same distance the event
horizon receded.
