Striking potential in the low-voltage mercury arc.—(1) Variation with vapor pressure. A tube with a thoriated tungsten filament and a nickel anode over a reservoir of mercury was heated in an electric furnace. When a constriction was placed in the tube leading to the pump so as practically to eliminate streaming of the vapor, and the temperature raised, the striking potential decreased to a minimum of approximately 5.3 volts for a pressure corresponding to a mercury surface temperature of 160°C and then increased. When no constriction was present, a similar curve was obtained except that on account of the streaming the pressure for a given furnace temperature was lower and hence the minimum was not reached below 320°C. If the fact that the vapor was fresher in the second case had any influence, it was not evident. (2) Variation with the distance away of mercury surface. When streaming was prevented, the same striking potentials were obtained for the same vapor pressures whether the surface was 5 cm or 80 cm from the arc. These results are somewhat at variance with those of Yao and suggest that further investigation is necessary.

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