  It is predicted that oscillations of temperature during propagation of third
sound in a thin superfluid film cause appearance of an alternating electric
field in the surrounding space (a peculiar non-stationary thermoelectric
effect). The magnitude of this field depends significantly on the substrate
type and the method of its coating. It is shown that the differential thermal
EMF (the ratio of electric potential amplitude to the film temperature
amplitude) can exceed such one in metals and reach $10^{-4}$ V/K.
