In a pulsed arc with mercury electrodes the average column field strength Eco was measured for different currents I, and the relationship [Formula: see text] was found. The variations of arc current and voltage with time indicate that the anode and cathode fall regions have a combined thickness of 3 × 10−6 cm, and an average field strength of 2 × 106 V/cm. The arc plasma is formed mainly from mercury vapor, rather than from the surrounding gas.