Philadelphia Paper - Deterioration of Nickel Spark-plug Terminals in Service (with Discussion)

The most commonly used material for terminals in spark plugs is commercial nickel wire, because of its relatively high temperature of melting, excellent heat conductivity, and slow rate at which the metal is oxidized, even upon continued heating at high temperatures. The grade commonly used for this purpose averages 97 per cent. nickel, the remainder being manganese, cobalt, iron, copper, with traces of other impurities always found in commercial nickel. A peculiar and interesting type of deterioration that occurs in these nickel terminals during the service life of the spark plug was recently brought to the attention of the Bureau of Standards. Commercial spark plugs vary greatly in their size and shape and the terminals differ considerably as to their relative size, shape, number, and arrangement with respect to one another. Although this defect was studied in detail in only one form of spark plug, the deterioration is a characteristic of the material, i.e., of nickel wire, rather than of the particular type of spark plug in which it was noted and studied. The mechanical features of the plugs, however, have a considerable bearing on the time required for the deterioration to become serious. The results of this study of the deterioration of nickel terminals in service should be of value to the makers of all types of spark plugs in which the terminals are made of this metal. CharacteR of Deterioration of WIres During Service Macroscopic Appearance and Properties of Changed Wire.—The nickel terminals in which the deterioration was studied were taken from spark plugs having a central terminal with the ground or side terminals attached firmly at both ends to the shell of the plug. In some of the plugs, two side terminals were used, one on each side of the central one. Although both terminals were found to have deteriorated to some extent, the attack of the central one was quite negligible compared to that of the side ones. These latter wires had developed, in service, transverse cracks that, in many cases, were as sharp and definite as a knife-cut. After a separation occurred, the breach widened by loss of material from the