MASSACRE: a Machine for Automatic Surface Sampling and Automatic Contact-Resistance Evaluation

Studies of the effects of environment on the mechanical and electrical properties of surface films on contact metals involve many measurements of properties, such as contact resistance, for a range of loads applied both with and without an associated surface traction (wipe). It is impracticable, and wasteful of effort, to do such work manually. The machine described does it automatically: ten loads are available and a plate two inches square can be tested at a thousand different places, each observation occupying eight seconds. The test sequence at each place is as follows. A probe is lowered gently on to the plate, and when stationary, the contact resistance is measured and classified automatically. The probe is then `wiped¿ and, again when stationary, a similar evaluation is made. This procedure is repeated for each of seven loads (10, 20, 50, 100, 200, 500 and 1000 g) or until one or other of the two sets of contact resistances falls within a specified range. The remaining three loads can be applied manually when it is necessary to explore a particular load interval in detail. Resistance measurements are made with a 4-terminal bridge arranged so that the open-circuit voltage at the source is about 40¿50 mV. A switching unit takes the place of the variable arm in the bridge, and substitutes, in turn, one resistor from a group of five. When the bridge condition passes through a balance as a consequence of bringing a particular resistor into circuit, the event is recorded on one of a group of five meters corresponding to the resistance classifications used.