Contact charging between metals revisited

The theory of contact charging between metals was established in 1951 by Harper and confirmed experimentally in 1975 by Lowell who measured the net charge after separation of pure metals in contact and related it to the contact potential difference and capacitance of the surfaces using a specialized measurement technique. It is generally assumed that these contact charges can be ignored for metals used in common industrial processes because of charge backflow on separation. A series of experiments carried out on commonly used industrial materials shows that net charge does exist on spherical metal balls rolling out of a metal tube into a Faraday pail. The experiments were carried out using three different tube materials (copper, brass, aluminum), five different ball materials (copper, brass, aluminum, steel, and stainless steel), for ball diameters ranging from 0.24 to 1.27 cm. The surfaces of the materials were only rinsed with water and dried. Analysis of the results shows general agreement with the Harper theory and confirms the presence of net charge on metals separated in this simulated industrial operation, i.e., moving metal parts off a conducting conveyor. This process is analyzed from the point of view of possible discharge hazards and the reasons why this phenomenon is not more widely observed in industrial processes are discussed.