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A simplified model is developed to calculate the temperature rise of the anode surface in vacuum arcs. The model predicts the threshold current for anode melting as a function of the properties of the arc and electrode materials. The calculated and measured currents agree well for six anode materials. In five of these cases an anode spot formed at the time calculated for melting to occur. It is concluded that for the geometry analyzed, anode spot formation is closely linked to anode melting and possibly to magnetic deformation of the molten surface. This lends support to the conjecture of Lafferty that localized heating of the anode can trigger spot formation.