Motion and Spectrum of Arc Cathode Spot in a Magnetic Field

The velocity of the cathode spot of a mercury arc at the junction between liquid and metal in a transverse magnetic field has been measured for magnetic field strengths between 0 and 20 700 oersteds. The approximate doubling of retrograde velocity at about 11 000 to 15 000 oersteds was followed by an additional rapid rise of velocity at about 15 000 oersteds. Spectra of the arc showed Hg ii and Hg iii lines at the stronger magnetic fields. Radiation from the cathode spot showed mercury lines and a continuous spectrum which is especially intense at the lines. Some Hg lines are broadened symmetrically and others asymmetrically. If the broadening is due to a Stark effect, the electric field strength in the cathode spot region is greater than 6×${10}^6$ volts/cm.