Development of resistance at a disturbed contact: with special reference to silver, copper and nickel-silver

The variation of resistance with time, for continuously mechanically disturbed base-metal contact in air, is characterised by a resistance maximum, followed by a minimum and a subsequent increase to disconnection. The significant factor is the number of times the contact has been disturbed. In principle, disturbance can involve sliding or normal impact, or both together. The effect of either occurring separately is broadly similar, but with sliding the resistance minimum corresponds to near-metallic contact, whereas with normal impact it does not. Coexistent impact inhibits the effect of sliding. The first resistance maximum is associated with polishing of the contact area, the following minimum with shattering of the area. A significant increase in hardness occurs after the minimum. The maximum can be stepped or double-peaked according to whetherthe initial films on the two contact surfaces finally fail separately or together. Disintegration of the contact area starts below the surface.