Effects of the metal plasma jet emitted from the cathode spot of a vacuum arc (or a unipolar arc) on the surrounding electric field variation are investigated. It is shown, using a 2‐D magnetogasdynamical model, that the cooling of the expanding metal plasma over the cathode surface leads to a strong variation of the electrical conductivity across the thermal boundary layer. This effect, which was ignored in previous theories of the cathode spot, coupled with the induced electric field caused by the plasma motion across magnetic fields which are caused by the arc current and the external currents, gives rise to a strong electric field at the cathode surface. Solving a mixed‐boundary‐value problem for the electric potential, it is shown that the induced electric field is consistent with the random and the directed retrograde motions of the cathodic arc spot.