The Behaviour of the Cathode Spot on an Undisturbed Mercury Surface

The cathode spot of transient mercury arcs has been studied by means of a Kerr cell apparatus capable of taking many superimposed photographs. Discussion is limited to times when the spot is moving with sufficient velocity not to disturb the mercury surface. It is found that for a given current the "spot" takes the form of a line, or broken line, of total length proportional to the current. This is also true for a spot in a magnetic field. If the current rises slowly, the length of the line increases proportionally to the current; but if the rate of growth of current is greater than a value lying between π × 10 7 and 2π × 10 7 amperes per second, then fresh spots form and spread out radially from the point of formation into lines moving with a maximum velocity of about 10 4 cm/sec. It is found that this velocity is never greatly exceeded whatever the experimental conditions. The current density is in excess of 10 6 amperes/cm 2 .