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The axial force carried by the expanding plasma plume from 50-250-A copper and aluminum vacuum arcs was measured using a pendulum whose axle was equipped with a rotary optical encoder. It was found that the force was a linear function of current. The electrode geometry was varied to find the maximum force. At maximum, the average forces per unit current were 21.4 dyn/A for aluminum cathodes and 36.1 dyn/A for copper cathodes. The Cu ion current fraction was measured to be 8.6%.