Stability of Low Pressure Mercury Arcs as a Function of Current

The persistence of an arc in an experimental tube with a pool type cathode has been studied. The mean life under carefully controlled operating conditions is determined by statistical analysis of a hundred or more observed lives and is taken as a measure of the stability. The dependence of mean life upon both the ambient temperature of the cathode pool and the current is determined. The arc is found to be most stable at an optimum ambient temperature. In the first observations the cathode spot was free; later observations were made with the cathode spot anchored in contact with a tungsten film. In both cases the results indicate that the mean life rises exponentially with the arc current in the ranges used. If I represents the arc current and L represents its average duration, the results suggest a relationship of the form L=F(p,x,y,z,…)exp (kI),where F stands for some functional relationship depending on the mercury vapor pressure p calculated from the ambient temperature, and upon other parameters such as x, y, and z, at present undetermined. For the proportionality constant k, the data give (2.46±0.06) ampere−1 when the cathode spot is free and (19.92±0.47) ampere−1 when the cathode spot is anchored. F(p, x, y, z, …) appears to be smaller with the cathode spot anchored than with it free, but the probable error in the function is relatively large.