Contact resistance is a hindrance to the flow of thermal, electrical, or kinetic energy at an interface between conductive materials. Such surfaces of contact can be formed by welding, soldering or simple mechanical contact.
Contact resistance is very important in the design, construction, testing and operation of electrical systems and thermal systems, because it causes energy loss and inefficiency in device operation and inaccuracy and unreliability in measurement.
Contact resistance can occur at the interface between different phases of the same material, where two pieces of the same material touch, or where two pieces of dissimilar materials touch. The resistance depends on many factors, including the actual surface area of contact (constriction resistance), the presence of oxides or other products of chemical reaction, absorption or adsorption, the cleanliness and the flatness of the two contact surfaces, differences in the conductivity of the materials, and tunneling resistance in thin-films.
One practical example of the important role of contact resistance in the operation of electrical circuits is in the operation of circuit breakers. A circuit breaker is like a switch that physically interrupts the flow of current in a circuit, usually to protect equipment and wiring systems against overcurrent. The ignition system of some internal combustion engines have contact breaker points that open and close rapidly in synchronization with the rotating engine crankshaft to deliver current with the right timing to a spark plug. The points can become fouled by oil or dirt, which acts as an insulator between the points and increases the contact resistance. The surface of the points can also be roughened by metal transfer from one surface to the other as a result of current arcing. Arcing may also cause formation of oxide layers on the point surfaces. Changes in the contact resistance of the points over time can cause mistiming of the ignition in the combustion chamber and poor engine performance. This is one reason that breaker-point ignitions have largely been replaced by electronic ignition systems in automobiles.