  We have studied the high temperature multiferroic cupric oxide using
polarized neutron diffraction as a function of temperature and applied electric
field. We find that the chiral domain population can be varied using an
external electric field applied along the b axis. Using representation analysis
we derive the incommensurate magnetic structure in the multiferroic phase. The
origin of the magnetoelectric coupling is consistent with models based on the
inverse Dzyaloshinskii-Moriya interaction, but is different from the simple
cycloidal mechanism.
