The recovery of a normal zone produced in a superconducting magnet following a mechanical disturbance depends on the heat transfer rate to the liquid helium cooling channels. One of the factors which affects the heat transfer to the channel is the amount of vapor accumulated in the channel during and following a disturbance. The present work is undertaken to determine the void fraction in a liquid helium channel and to study the effect of the amount of vapor in the channel on the heat transfer characteristics. Heat pulses of 30 to 300 ms duration are introduced into the simulated conductor, and observations are made of the resulting temperature response and of the vapor fraction in the channel. The vapor fraction is determined by means of a capacitance bridge which responds to the small difference in dielectric constant between liquid and vapor helium. For completeness steady state measurements of temperature and vapor fraction are also reported.