Industry instrumentation : Units and conversion tables for process control  Instrumentación industrial : Unidades y tablas de conversión para el control de procesos


The resistivity ρ of a material is the resistance to current flow between the opposite faces of a unit cube of the material (ohm per unit length). The resistance R of a component is expressed by where l is the length of the material (distance between contacts), and A is the crosssectional area of the resistor; l and A must be in compatible units. The following Table gives the resistivity of some common materials. The resistivity ρ is temperature dependant, usually having a positive temperature coefficient (resistance increases as temperature increases), except for some metal oxides and semiconductors which have a negative temperature coefficient. The metal oxides are used for thermistors. The variation of resistance with temperature is given by where
The variation of resistance with temperature in some materials (platinum) is linear over a wide temperature range. Hence, platinum resistors are often used as temperature sensors. 
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