Cathode Dark Space and Negative Glow of a Mercury Arc

Abstract

Means were evolved to observe the cathode dark space of the mercury arc, the object being to measure the thickness and evaluate therefrom the voltage gradient at the cathode to distinguish between field and emission theories of electron liberation. A magnetic field transverse to the arc drives it in the opposite direction to the force involved. This wrong way motion made it possible to race the arc spot over smooth mercury while ions, electrons, and vapor were blown rearward. Photomicrographs showed a negative glow, its image in the mercury, and a space between; evidently twice the dark space. A one-ampere arc had a dark space of 0.001 cm; a hundred times too large for the field theory, and causing excessive space charge limitation of current unless compensating ionization occurs throughout said space. Phenomena in the negative glow must cause the needed ionization and also intensive electronic bombardment of the cathode. This is assumed to cause cumulative excitation of the liquid resulting in emission of electrons and light. A continuous spectrum originates---within the limits of measurement---at the liquid.