Energy balance, space charge, and thermionic equations have been combined to provide a description of the mechanism of the cathode in an ac arc, and the quantities required for solution determined experimentally. The experiments were carried out on barium‐activated electrodes in xenon at 1.4 atm and extended over a range of 25–75 amps rms. The work function was found to be 1.77 v at zero‐field and reduced to 1.67 v under Schottky effect of the space charge field. The work function did not change with current, but the Richardson‐DushmanAfell, as the current was raised, to a value of approximately 0.1 at the highest load. The cathode fall varied with phase angle and showed a maximum of 1.2 v at maximum current. The component of current at the cathode carried by ions reduced from 12% to 6% as the load increased.