The sliding contact of graphite and copper

Experiments with well-developed sliding contacts of graphite and copper have shown the marked influence of atmospheric and electrical conditions on the kinetic friction and contact resistance. Dry oxygen at low pressures appears to be lubricating for currents of any magnitude flowing in either direction through the contact. However, at higher pressures of this gas it is no longer lubricating and its effect on the friction and resistance is markedly dependent upon the direction of the current. Water vapour alone is found to be lubricating, but in company with oxygen its action is complex and it modifies the effect of oxygen profoundly. It lubricates currentless and negative brushes, but increases the kinetic friction of positive brushes, probably as a result of its stabilization of the contact interface. Increase in the pressure of dry oxygen always increases the contact resistance of positive brushes, but an initial increase at moderate pressure with negative brushes is followed by a marked decrease at higher pressures. Water vapour alone has only a slight effect on contact resistance, but it modifies the effect of oxygen quite remarkably, especially in the case of positive brushes. Again its action is complex, but in the main it tends to reduce the contact resistance of positive brushes and to increase that of negative brushes.