Measurement of the resistance of light-duty electrical contacts

In light-duty contacts failure is caused less frequently by erosion than by high resistance due to contamination of the surface. The best guide to the degree of contamination of a contact is obtained by measuring its resistance in such a way as to avoid damaging the surface film, even if the normal use of the contact may entail such damage. A 4-terminal method of measurement should be used in order separate the resistance of the contact from that of the switching device as a whole and its connections to the measuring network. The conditions in which the separation is physically meaningful are considered, and a simple calculation indicates where the connections should made in a typical case. Theoretical arguments, supported by experiment, are given for limiting the test voltage to 10mV. The current need not be limited provided that the circuit is de-energized during make and break prevent damage to surface films by inductive voltage peaks. If this not done the current must be limited to a very small value, with serious loss in sensitivity.