The Formation of Metallic Bridges Between Separated Contacts

Low resistance bridges were formed between gold, steel and carbon electrodes having separations of 2-70×{}10^-6 cm by applying voltages less than the minimum sparking potential. For a given pair of electrodes the field required to form the bridges is a constant and is 5-16×{}10⁶ volts per centimeter. Measurements of the temperature coefficient of resistance of the bridges identify them as consisting of the material of the electrodes. A study of their resistance as a function of the displacement of one of the electrodes shows that they may be pulled out as well as crushed. At voltages less than those required to form the bridges, field currents exist. These increase rapidly as the field is raised and attain a value around 10^-10 ampere before the bridges are formed. Calculation of the maximum electrostatic stress on the electrodes at the time of breakdown gives a value 0.05 to 0.0005 times the tensile strength of the electrode material at room temperature. The field is locally higher than that calculated because of surface roughness and the tensile strength is probably lowered by the local heating known to accompany field currents. The data therefore indicate that electrostatic force pulls material from the electrodes to bridge the gap.