The contact surface

The bodies in contact are called contact members, or, simply contacts when no misinterpretation is likely. According to the roles played by them in the electric circuit they are called electrodes or anodes and cathodes. The force, P, pressing them together is the (mechanical) contact load. Material surfaces are never perfectly smooth, and if the contact members were ideally hard they would never touch each other in more than three points. As a matter of fact, the contact material is always deformed, elastically or plastically, under the action of the pressure. Consequently, the initial touching points develop into small areas, and new contact spots set in. The sum of all these small contact areas makes up the load-bearing contact area, A b , which is much smaller than was formerly supposed. For example, the load-bearing contact area between a brush and a slip ring can be hundreds or thousands of times smaller than the polished area; the latter area we call the apparent contact area, A a , cf. §§ 1 and 4.