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The behavior of the contacts of the Panel Bank Switch, in which a phosphor-bronze brush slides into position over brass terminals, is studied in a simple laboratory apparatus in which the mechanical operation of the switch is simulated. Contact resistance between brush and terminal is measured as a function of the number of brush-terminal encounters. Terminal wear and static friction are measured concurrently in selected cases. Contact resistance behavior with and without lubrication by U.S.P. petrolatum, chosen after extensive corrosion tests, is studied. Sparing and copious lubrication, the effect of additives, and substitution of silver for one or both contacts, are studied. It is concluded that organic contamination or lubrication of the contacts, in conjunction with slide, lead to the development of a coherent film, probably oxide, bonded to the wear track by products developed as a result of chemical reactions induced by sliding, a process known as mechanical activation. If oxide is prevented from forming or if the products of mechanical activation are dispersed, the rate of development of an insulating film can be greatly reduced.