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The effect of a vacuum environment on welding of Cu-Bi (0.3%) electrical contacts has been investigated. The experiments have been planned in order to differentiate between the effects of cold welding, diffusion welding and percussion welding. Cold welding has no affect on newly prepared electrodes, but after the electrodes have been arced appreciably a small residual cold weld force is experienced. The diffusion weld strength is strongly dependent upon the contact force, the electrode temperature and the time in contact. The strongest weld forces are measured in the percussion weld experiment where the electrodes experience a high current prestrike arc as they close. It is also found in this study that the weld force decreases rapidly after the electrode faces are exposed to the residual gases in the vacuum. These results are discussed in terms of the chemisorption of residual gases and the real area of electrical contact. The effects of the change in nature of the electrode surfaces are discussed and a satisfactory qualitative explanation of the results is reached.