The electrostatic component of the force of sliding friction

For jerky motion due to relaxation oscillation to occur, the. {friction, velocity} characteristic of sliding must exhibit a non-linear falling section and a minimum. A similar type of characteristic belongs to any electrical discharge device with a practically infinite resistance below a definite breakdown potential at which it becomes converted into a conductor. It is suggested that, due to contact electrification, the electrostatic component of the force of sliding friction assumes appreciable proportions when the boundary layer has dielectric properties; thus cycles of slow charging and rapid discharging occur, and cause relative motion to proceed by cycles of slow sliding and rapid slipping when the average velocity, determined by the rate of propulsion of one friction element, is smaller than the velocity for which the frictional force assumes the smallest value. Experiments are presented to demonstrate that jerking decreases when the dielectric breakdown strength of the lubricating material is diminished, and that with the same lubricant jerking decreases as the rate of propulsion is increased. In these experiments, solid metal cylinders of 5.33 cm. nominal diameter were forced through hollow steel cylinders of slightly smaller bore, with various lubricants, in a 15,000-kgm. wt. recording Buckton testing machine which was operated slowly and at a uniform rate. The electric discharge current which accompanied jerking, even when both friction elements were made of steel, was observed when the friction elements were insulated from the Buckton machine and connected to a ballistic galvanometer. In accordance with the electrostatic interpretation of jerky motion due to relaxation oscillation, the surfaces remained undamaged and Amontons' law was obeyed when heavy jerking took place, whereas the relative motion of the two friction elements proceeded smoothly when the surfaces became severely torn, and the tangential force increased at a rate more than proportional to the radial force.