The problems associated with contacts between nominally clean metallic surfaces approaching and separating normally (as distinct from continuously sliding) may be divided into two groups according to whether the path between the surfaces is metallic or gaseous. The paper is concerned with the first of these groups. The field of interest may be further sub-divided, as it includes both the phenomena associated with nominally static contacts and those relevant to separating contacts up to the instant when the metallic path between the contacts ceases to exist.The first part of the paper is, in some respects, an extension and development of Holm's work, although originally done with little or no reference to the earlier results. All the effects encountered in the change of resistance with current and mechanical pressure are shown to be predictable on the basis of the existence of contact spots, and new contributions are made in several directions. The extents of resistance changes are directly related to the mechanical pressure. A fresh technique is described, by means of which the existence of the spots may be demonstrated and a lower limit assigned to their number. The influence of the rate of current loading is examined, and further verification of the plastic character of the yielding process is furnished. The measurements described relate to one particular contact material, a platinum-iridium alloy, and to one surface finish. They cover much wider ranges of pressures and voltages than are encountered in practice, and permit identification of all the significant events observable in a normal laboratory atmosphere, i.e. without the employment of vacua or special surface-cleaning techniques. This range of observations is bounded only by limits at which the effects cease to be those relevant to a clean metallic contact. One limit, attributable to surface films, not necessarily due to tarnishing, is encountered at very small pressures and voltages: the other appears at higher voltages which, if exceeded, result ultimately in glowing and fusion of the contact surfaces.The second part of the paper is concerned with the unequal wear of the two members of a contact pair (“selective erosion” or “unbalanced erosion”): this is frequently accompanied by a gain of material by one member at the expense of the other (“material transfer” or “transfer”). In severe cases one member may develop a large pip while the other produces a corresponding crater: the contacts may then lock together. Hitherto, experience has suggested that such pips and craters occur in a random manner and that neither seems to be associated with a particular contact polarity. The work described presents a new and simplified approach to the problem. It is suggested that, in general and perhaps more especially when quenching is permissible, unbalanced erosion results from, or can be made to result from, two main causes: first the molten metallic bridge joining the contacts when only partly separated, and secondly the arc. The sense of arc erosion is always the same, independent of the metal, whereas that of bridge erosion depends on the sign of the Thomson coefficient of the metal near its boiling point. Thus metals for which the senses of the bridge and arc erosion are the same can only exhibit one sense of erosion: but those for which they are opposite, can exhibit both senses, or even none at all, depending on which effect predominates due to appropriate circuit conditions. This leads to the idea of alloys so designed as to possess a zero Thomson coefficient near their boiling point, which would therefore give equal bridge erosion of both contact members. Progress has been made in the development of such alloys. The remaining unbalanced arc erosion would then be reduced as far as possible by the use of an appropriate quench. Such alloys would, of course, have to satisfy all the conventional requirements for contact materials and, if possible, one more: even with a quench, the possibility of slight residual arcing cannot be neglected, so that it would be desirable, when selecting metals for the development of balanced bridge erosion (“B.B.E.”) alloys, to do so from those which do not readily support an arc.