The formation and rupture of molten metal bridges in electrical contacts

Abstract

The bridge of molten metal which is formed between the electrodes of an electrical contact is of considerable interest from a fundamental physical point of view. It is also of practical importance as the cause of erosion and failure of such contacts, which is due to the transfer of metals from one electrode to the other after repeated operations of the contact. The theory of the formation, development and rupture of contact bridges is discussed, and an experimental study of the properties of bridges between slowly opening contacts is described. Observations have been made on bridges in the ordinary atmosphere andin vacuo. The method of producing bridges for visual examination in air andin vacuo has been applied to an investigation of bridges of the noble metals, and of the readily oxidizable base metals. The temperature in a bridge can reach very high values, approaching the boiling point of the metal, with a potential difference of only one to two volts between the electrodes. Theoretical predictions of the shape of molten metal bridges as controlled by the temperature distribution and by surface tension and electrodynamic forces are compared with the shapes actually observed. Photographs of bridges showing the agreement between theory and experiment are given. The potential difference between the electrodes when the rupture of the bridge takes place, — the rupture voltage, has been measured for a number of metals: Pt, Pd, Ag, Au, Cu, Fe, Ni, W, Mo and Sn. The results are discussed in relation to theories of the cause and the mode of rupture and to the electrical and thermal properties of metals at high temperatures.