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The resistance of certain electrical transition interfaces involving plane, nonplated aluminum contact surfaces increases with service. Analysis of resistance characteristics indicates formation of the contact interface during connection installation produces systematic disruption of the surface film. Interfacial shear strain -- relative movement--between the contact members is identified as the efficient cause of the increase in resistance. If the selected combination of contact surfaces and members for an application does not assure adequate interfacial fixity, the introduction of a shear component of contact force--parallel to the plane representing the interface--may destroy or, if destroyed, may reestablish elemental electric contacts, depending upon the orientation of the shear component. Test data include contact resistances of interfaces involving Al and Cu conductors as well as Ag, Cd, Ni, and Sn platings.