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Simultaneous measurements Were made of the friction force and electrical contact drop for small metal fiber brushes at high current densities (12.4 MA/m2). Comparative testing was performed with both polarities of copper, silver-plated copper, and silver fiber brushes on copper and On silver-plated copper slip rings in humidified carbon dioxide. Friction increased from 0.3 to 1.4, while the contact drop decreased as more silver was added. At a contact force of 0.23 N and a sliding speed of 12 m/s, minimum total power loss was found for the bare and the silver-plated copper fibers on the copper sllp ring. However, the optimum material selection will depend on current density, sliding speed, and contact force Used for a specific application. Maximum test duration has been extended to almost 500 h, with indications of slight improvement of both contact drop and wear rate as the operating time is increased. Dimensionless wear rate was only 1.9 x 10-11. With added cooling capability, successful Shortterm operation has been achieved with current density in the individual fibers as high as 100 MA/m2 (65 000 A/in2).