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A theory of electrode controlled arc expansion is developed in terms of the finite time it takes for the current path in the electrodes, to change from the highly constricted state during the molten bridge stage, to the less constricted state of the fully developed arc. This theory gives satisfactory agreement for vacuum arc expansion rates when freshly prepared electrodes are used. The expansion rates for the arc after the electrodes have been arced once, however, are much slower. By considering small changes in the work function of the surfaces of the cathode we give a qualitative explanation of this slower expansion.