Electrical contacts Electrical contacts An electrical contact is an electrical circuit component found in electrical switches, relays, connectors and circuit breakers. Each contact is a piece of electrically conductive material, usually metal. When a pair of contacts touch, they can pass an electrical current with a certain contact resistance, dependent on surface structure, surface chemistry and contact time; when the pair is separated by an insulating gap, then the pair does not pass a current. When the contacts touch, the switch is "closed"; when the contacts are separated, the switch is "open". The gap must be an insulating medium such as air,
Coal measures Coal measures The coal measures is a lithostratigraphical term for the coal-bearing part of the Upper Carboniferous System. The Coal Measures Group consists of the Upper Coal Measures Formation, the Middle Coal Measures Formation and the Lower Coal Measures Formation. The group records the deposition of fluvio-deltaic sediments which consists mainly of clastic rocks (claystones, shales, siltstones, sandstones, conglomerates) interstratified with the beds of coal. In most places, the coal measures are underlain by coarser clastic sequences known as Millstone Grit, of Namurian age. The top of the coal measures may be marked by an unconformity, the overlying rocks being