Sparking Potentials at Low Pressures

At low pressures where only a few electrons produce ions, the number of electrons set free at the cathode by the impact of the positive ions formed in the gas is of primary importance in determining the minimum sparking potentials. Using the data for the efficiency of ionization by electrons of various velocities, and recent observations of sparking potentials in air, we may deduce the number of electrons set free by the impact of a positive ion on a nickel cathode for any velocity. This number increases with the velocity of the ion from approximately 4 at 3000 volts to 85 at 30,000 volts. These numbers are much greater than those given by direct observations. The explanation is suggested that the spark originates at small spots where the emission is especially abundant, whereas the direct observations of secondary electron emission give an average value for the surface.