  We explore the force generation and surface interactions arising when
electric fields are applied across fluid films. Using a surface force balance
(SFB) we measure directly the force between two electrodes in crossed-cylinder
geometry across dielectric and electrolytic fluids. In the case of dielectric
films the field between the electrodes exerts a force which can be well
explained using classic expressions and with no fitting parameters. However
when the electrodes are separated by a film of electrolyte, an alternating
electric field induces a force which diverges substantially from the calculated
static response of the electrolyte. The magnitude of the force is larger than
predicted, and the interaction can switch from attractive to repulsive.
Furthermore, the approach to steady state in electrolyte takes place over
$10^2$ -- $10^3$~s which is very slow compared to both the charging and viscous
timescales of the system. The non-trivial electrolyte response in AC electric
fields, measured here directly, is likely to underlie several recent reports of
unexpected and bifurcating forces driving colloids in AC fields. Our
measurements suggest ways to control colloidal and soft matter using electric
fields, as well as providing a direct measure of the length- and time-scales
relevant in AC electrochemical and electrokinetic systems.
