Explain Contact resistance.

Contact resistance: It is measured as the voltage drop from tail to tail of the mated contacts along with specified current flowing through the contact and is given in milliohms. The total resistance offered through a contact depends on the bulk resistivity of the contact material, actual surface area and mechanical wear of the contacts and environmental effects. To acquire low contact resistance this is essential that the contact must be a fine conductor of electricity. The contacts must have good mechanical resistance against wear due number of mechanical operations. The contacts are made by using alloys that have combination of mechanical properties, very fine electrical and thermal conductivity and resistance to corrosion. Usually used alloys are Beryllium copper, spring brass, low leaded brass and phosphor bronze. Plating of the contact metal is completed to prevent deterioration of contacts chemically and mechanically and to acquire good surface conductivity. Commonly utilized plating materials are as silver, copper, gold, tin (Pb) and nickel.