  A force-free surface (FFS) ${\cal S}$ is a sharp boundary separating a void
from a region occupied by a charge-separated force-free plasma. It is proven
here under very general assumptions that there is on ${\cal S}$ a simple
relation between the charge density $\mu$ on the plasma side and the derivative
of $\delta=\E\cdot\B$ along $\B$ on the vacuum side (with $\E$ denoting the
electric field and $\B$ the magnetic field). Combined with the condition
$\delta=0$ on ${\cal S}$, this relation implies that a FFS has a general
stability property, already conjectured by Michel (1979, ApJ 227, 579): ${\cal
S}$ turns out to attract charges placed on the vacuum side if they are of the
same sign as $\mu$. In the particular case of a FFS existing in the
axisymmetric stationary magnetosphere of a "pulsar", the relation is given a
most convenient form by using magnetic coordinates, and is shown to imply an
interesting property of a gap. Also, a simple proof is given of the
impossibility of a vacuum gap forming in a field $\B$ which is either uniform
or radial (monopolar).
