The Mechanism of the Electric Spark

Abstract

Meek's criterion for the formation of a spark discharge in a gas is criticized. An error in the derivation of his formula for the radial field about an electron avalanche channel is noted, and the value adopted for this field is shown to be too small to draw the postulated number of electrons into the channel from the space outside. The importance attached to the radial field is found to be misplaced because of the above and because a criterion based solely on the total number of ions produced in an avalanche gives results for sparking potentials for different spark lengths and for a gas at different pressures in agreement with those obtained by Meek's method. Both methods give results which, using the values assigned to the constants, fall more and more below the experimental values of the sparking potentials as the length of the spark gap, δ, is increased until at 20 cm the deviation is over 7 percent. To bring the computed value for δ=20 cm into agreement with the experimental value the constants in the equations would have to be increased by a factor 2×10¹⁶ over the values which give agreement between formulas and experiment for δ=1 cm. The dominant exponential term in each formula is based on the incorrect assumption that the field is not affected by the space charges in the avalanche. That a large amount of ionization, postulated by Loeb and Meek, is produced by a high potential wave which travels through the channel as soon as the positive streamer reaches the cathode is questioned because the fields found in such waves are too small to produce ionization by electron collision in air at atmospheric pressure.