  We report on the measurement of a Larmor frequency shift proportional to the
electric-field strength for $^{199}{\rm Hg}$ atoms contained in a volume
permeated with aligned magnetic and electric fields. This shift arises from the
interplay between the inevitable magnetic field gradients and the motional
magnetic field. The proportionality to electric-field strength makes it
apparently similar to an electric dipole moment (EDM) signal, although unlike
an EDM this effect is P- and T-conserving. We have used a neutron magnetic
resonance EDM spectrometer, featuring a mercury co-magnetometer and an array of
external cesium magnetometers, to measure the shift as a function of the
applied magnetic field gradient. Our results are in good agreement with
theoretical expectations.
