1969 Volume 38 Issue 1 Pages 72-78
An arc with straight polarity is formed between a 2% thoriated tungsten electrode and a water-cooled copper plate in argon atmosphere of 18 to 760 mm Hg, and the electrical characteristics of the arc is examined under the following conditions.
Pressure of atmosphere P : 18 to 760 mm Hg
Arc current I : 250, 450 A
Distance between electrodes L : 1 to 25 mm
The facts revealed are as follows.
(1) At reduced gas pressures a radiant sphere, so called "cathode zone", is observed in the arc just below the cathode. Though it is brighter and has higher energy density than the arc column, its potential gradient does not differ from that of the arc column. The radius of the cathode zone increases with decreasing gas pressure and is the same order of the mean free path of an electron at P=32 mm Hg and I=10 A.
(2) With the decrease in gas pressure, the arc is extended both upwards and sideways and its brightness decreases. Moreover the plasma stream from the cathode to the anode is weakened since electromagnetic pinch force decreases.
(3) Potential gradient of the arc column E decreases linearly with the decrease in log P. The measurement shows that the difference between E near the cathode tip and E at the other place of the arc column is reduced with the decrease in gas pressure.
(4) The sum, VA+K, of cathode drop VK and anode drop VA increases with the decrease in gas pressure. VA+K increases from 7 to 14 volts as P decreases from 760 to 18 mm Hg at I=250 A.