Motion of an Arc in a Magnetic Field

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Abstract
A magnetic field transverse to an arc tends to move the cathode spot in either the direction of the electromagnetic force (the Amperian sense) or in the opposite direction (the retrograde sense). The theory of A. E. Robson and A. von Engle assumes that an arc, bent over in the Amperian direction forms a loop in which the magnetic field of the arc opposes the applied field to yield the retrograde motion. Calculations herein presented indicate that the arc field is too weak to uphold the theory. Retrograde motion proceeds continuously against a blast of vapor, while Amperian motion proceeds in the direction of the blast and is often discontinuous. These phenomena are recalled in giving an alternative explanation of the data. Examination of the ``ball of fire'' discharge, which can display retrograde motion, shows the excitation phenomena in the region between dark and bright plasma yields information about retrograde motion. Studies of the corresponding dark space and negative glow of an arc and a better knowledge of electron liberation at the cathode give promise of yielding information about the retrograde motion.

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