Arc attachment and fall voltage on the cathode of an ac high-pressure mercury discharge

By observation of arc attachment on the cathode of an ac high-pressure mercury discharge, a transition from a diffuse mode at low current to a spot mode at higher current was found. The transition was accompanied with a sudden decrease in the arc voltage of about 10 V. At decreasing current there is a reverse transition with a voltage increase. A recent cathode-fall model shows quite a similar behaviour. Further, the electrode sheath voltage (essentially the sum of anode and cathode fall) was measured and compared with the calculated cathode fall. The obvious differences especially during the current rise were ascribed to the anode fall which should attain highly negative values in the order of 10 V.