  When a high voltage (~30 kV) is applied to a capacitor whose electrodes have
different physical dimensions, the capacitor experiences a net force toward the
smaller electrode (Biefeld-Brown effect). We have verified this effect by
building four capacitors of different shapes. The effect may have applications
to vehicle propulsion and dielectric pumps. We review the history of this
effect briefly through the history of patents by Thomas Townsend Brown. At
present, the physical basis for the Biefeld-Brown effect is not understood. The
order of magnitude of the net force on the asymmetric capacitor is estimated
assuming two different mechanisms of charge conduction between its electrodes:
ballistic ionic wind and ionic drift. The calculations indicate that ionic wind
is at least three orders of magnitude too small to explain the magnitude of the
observed force on the capacitor. The ionic drift transport assumption leads to
the correct order of magnitude for the force, however, it is difficult to see
how ionic drift enters into the theory. Finally, we present a detailed
thermodynamic treatment of the net force on an asymmetric capacitor. In the
future, to understand this effect, a detailed theoretical model must be
constructed that takes into account plasma effects: ionization of gas (or air)
in the high electric field region, charge transport, and resulting dynamic
forces on the electrodes. The next series of experiments should determine
whether the effect occurs in vacuum, and a careful study should be carried out
to determine the dependence of the observed force on gas pressure, gas species
and applied voltage.
