A commonly quoted derivation of the well-known formula for the current induced by a charge moving between electrodes is based on an energy-balance equation which is incomplete. The result is erroneous if this method is extended to include the effect of stationary space charge. The current due to a small mobile charge can be used to map the field in a dielectric material, but one of the two calibrating formulae found in the literature is incorrect, since it relies on the faulty derivation.