The Bridge Erosion of Electrical Contacts. Part I.

Abstract

Bridge erosion is the transfer of metal from one electrode to the other which occurs when an electric current is broken in a low voltage circuit which is essentially purely resistive. It is associated with the bridge of molten metal formed between the electrodes as they are pulled apart, and more specifically with the ultimate boiling of some of the metal of this bridge before the contact is finally broken. This paper is concerned with fundamental studies of this molten bridge and with empirical measurements of the transfer of metal. From known physical constants one calculates that, when the melting point is reached at a current I, the diameter of the area of contact of two electrodes is about 1.5×10⁻⁵ I cm for silver, copper, or gold, and about 7.5×10⁻⁵ I for platinum or palladium. When the maximum temperature of the molten bridge reaches the boiling point the mean bridge diameters are roughly 4×10⁻⁵ I and 20×10⁻⁵ I respectively. Experimental tests have been made of relations entering into these calculations. On breaking a contact about 6×10⁻¹⁴ I³ cm³ of metal is transferred from the positive to the negative electrode. This represents about 100 percent of the volume of the molten bridge for silver, copper, or gold, and about 0.5 percent for platinum or palladium. The amount of transfer can be decreased, and even its direction reversed, by heating the negative electrode, and by other means. Calculations have been made of temperature distributions in the neighborhood of a contact and a theory has been developed to account for the reversal of direction of transfer. The theory of the fundamental mechanism has, however, been reserved for later publication.