Resolving the electrode fall spaces of electric arcs

The measurement of the cathode and anode fall in potential in arc discharges between metal electrodes in gases at atmospheric pressure has presented considerable difficulties because of the small extent of the fall spaces. For the same reason there are no satisfactory observations of the thickness of these spaces. Early mirror oscillograms of te voltage of arcs of decreasing length indicated the existence of a minimum voltage, necessary to maintain an arc, and its value was taken to be equal to the sum of cathode and anode falls in potential. Attempts to determine these individually either by using different substances for the electrodes or by a moving probe have met with only limited success. In order to separate the electrode falls in a single experiment, a high speed cathode ray oscilloscope is used. The arc having been first established, its anode is driven towards the stationary cathode while the variation of arc voltage with time is recorded as the arc length is reduced to zero. Oscillograms show two discontinuities in the voltage trace which correspond to the anode and cathode falls. From the duration of the steps, and the known velocity of approach of the anode, approximate values of the thickness of the fall spaces are determined. Arcs between electrodes of tin, copper, graphite, and tungsten are investigated in argon, nitrogen and air, mostly at atmospheric, but also at reduced, gas pressures. At one atmosphere the cathode falls observed are between 11 and 15 V for currents of 10 A and above, whereas the anode falls lie in the range of 2 to 5 V. Fluctuations in arc voltage are found to originate mainly at the cathode. The cathode fall for tin in argon is constant below 30 A but slowly increasing at larger currents. The anode fall for graphite is independent of current up to 60 A. The thickness of the anode fall space in metal vapour arcs is of the order 10-2 to 10-4 cm depending on the gas. The thickness of the cathode fall space does not exceed 4 x 10-6 cm, a value consistent with the excitation theory of vapour arcs.