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The interface between two contacting metals, neither of which is noble, can be established under conditions such that the electrical resistance is essentially the same as for clean metals. In addition, this behavior has been observed over extended periods of time in chemically hostile environments. Some basic physical properties of contacts are applied for a qualitative understanding of these results. Surface topographical studies have shown that the real load bearing area in a local indentation is never less than 50 percent of · the nominal area of contact. The nature of the bulk deformation during the loading of the interface and the behavior of multiple electrical contact spots ensure that, even in the presence of insulating films, the fraction of true area in electrical contact is sufficient for good conduction. The application of percolation theory to the problem of sealing a contact against ingress of corrosive atmospheres predicts that roughly 44 percent of randomly placed real contact area is a threshold value, beyond which open pathways for diffusion of the external atmosphere become blocked. Since the true contact area exceeds this threshold, such interfaces will be gas-tight.