The Mercury Arc Cathode

Conduction electrons in the liquid at the arc spot are considered to be heated to about 4000°K by electronic bombardment from the vapor while the atomic temperature of the liquid is near 150°C. Thermionic emission apparently yields the arc current plus that returning from the vapor. An arc spot was driven around a circular mercury track by a radial magnetic field. The spot traveled opposite to the ponderomotive force for fields below approximately 5000 gauss with velocity proportional to field strength and showed a negative temperature coefficient. The Righi-Leduc effect considered as a thermomotive force operative in the hot layer of conduction electrons was assumed to drive the spot. The velocity was evaluated in terms of the vertical thermal gradient and compared with the observed, yielding a gradient of about 5×${10}^8$ degrees cm. Arcs were extinguished by current pulses of about ${10}^{-7\mathrm{}}$ sec. in the same direction as the arc current. This gave means for calculating the total energy of the hot electronic layer, and finally the vertical thermal gradient in good agreement with the value above. The Thomson heat stops losses along this gradient, and is apparently the factor causing the characteristic arc current density.