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Measured erosion rates are given for water-cooled oxygen-free high-conductivity copper cathodes over which arcs of 45ߝ800 A have rotated at speeds between 30 and 300 m/s. Contrary to the expectations of earlier workers, the erosion rates donot fall as arc velocity is increased, but, in some cases, they rise considerably. The variation of erosion rate with arc current has been examined using two different bases: (a) minimum erosion rate for any arc velocity or water-cooling flow rate, and (b) constant arc velocity. On the first basis, the erosion rate is proportional to I1.15, and on the second it is proportional to I1.2, but this only holds if the cathode roots can be spread uniformly over the whole arcing surface, avoiding a high-erosion mode with damage concentrated in a narrow region, which can occur at high currents.