Contact resistance is often measured to determine the condition of electrical, circuit board, or other kinds of cable connections. Typically dependent on the shape of the contacting surfaces and layers used for insulation, this type of resistance generally describes the flow of electricity, or heat, from one place to another. Parts with a larger surface area usually have low contact resistance, while those with relatively smaller surfaces tend to have a higher resistance. Gold and other soft metals usually have a higher surface area connection, so are often used in contact pads.
When two components are in contact with one another, the flow of electricity, and even heat in the form of thermal contact resistance, is often affected by microscopic phenomena in the material. Certain regions of metal, for example, are in direct contact and are fully conductive, while other areas can have a higher resistance and only partially conduct. Non-conducting regions are sometimes present as well.
Electrical contact resistance can be elevated if the connections between two plates, or a surface and a probe, are not tight enough. If joints need to be bolted together, a lack of tension can also affect conduction, while erosion of contacts or their surfaces is often a cause for high contact resistance. Contamination by dirt or other compounds can be a factor as well. The contacts on jumper cables often corrode, preventing electricity from being conducted between one automobile battery and another. Corrosion of fuse contacts can affect the device’s performance also.
If there is a high level of current flowing through a device, and the contact resistance is high enough, heat can build up and pose additional hazards. Test equipment such as an ohmmeter can be used to determine the level of contact resistivity in a connection. When performing this type of testing, one must often account for the metallic contact surface area, as well as whether the connection is soldered, clamped, bolted, or welded. Also, the pressure at which the contact is held together is usually considered.
Normal contact resistance is usually identified by manufacturers of the equipment being tested. Proper measurement ranges are often described on product manuals or sometimes labeled directly on the devices. A contact resistance measurement is generally important for monitoring the condition of power transmission lines. It is also sometimes performed to characterize fluid pressure, which can behave much like an electrical voltage, between different flow channels.