A technique is described for measuring the potential within a plasma by monitoring the time‐of‐flight of a beam of electrons directed through the plasma. The electron beam, intensity modulated at 30 Mc, is accelerated to an initial velocity determined by its cathode potential. The velocity then varies from this initial value as the potential along the flight path varies, changing the arrival time of the 30‐Mc signal at the collector. The phase of the collector signal is continuously compared to that of a reference signal derived from the intensity modulator. The phase comparator has an output which is linearly proportional to the phase difference between the two signals. The sensitivity of the technique depends principally on the energy of the electron beam and the response time of the circuits. Using 900‐v electrons, it is possible to measure a potential of 2 v along a 10‐cm path, or follow changes of this magnitude which occur in a microsecond. The sensitivity can be raised to look at potentials which change more slowly. Under the operating conditions used, 2 v along 10 cm represent a phase change of about 0.1°, or a time change of about 1×10−11sec.