Constricted or diffuse arcs rotating in high magnetic fields in air at atmospheric pressure

Experiments are described in which d.c. arcs of 200, 500 and 2100A were caused to rotate in air at atmospheric pressure between concentric copper electrodes by an axial magnetic field of up to 1.75Wb/m2. Measurements of arc rotational speed have been made using both optical and magnetic methods. It has been found that the use of optical methods alone might lead to the supposition that the arc became diffuse instead of constricted at the high velocities which are consequent upon high magnetic fields, whereas the magnetic method showed that no change occurred in the type of arc over the whole range. Other evidence that the arc remained constricted is given. It is concluded that arcs which were previously thought to have become diffuse in air under similar conditions were most probably in fact normal constricted arcs, and that the afterglow due to active nitrogen is the most likely source of error when optical methods are employed with arcs under conditions of this kind.