The cathode spot of transient arcs using the liquid sodium-potassium alloy for cathode has been studied by means of the Kerr cell apparatus capable of taking a sequence of photographs. It is found that for a given current the spot takes, as in the case of mercury, the form of a line or broken line of total length roughly proportional to the current. This is also true for a spot in a magnetic field. Again as for mercury, if the current rises slowly the length of line increases proportionally to the current If the rate of growth of current is greater than a value lying approximately between 10⁸ and 2 × 10⁸ amp/sec., then fresh spots form and spread out radially from the point of formation into thin semicircular or circular lines moving with a maximum radial velocity of about 10⁴ cm/sec. The apparent current density of emission lies between 2.5 and 5 × 10⁶ amp/cm², and consequently is higher than for mercury.