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


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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 cross-sectional 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


  • RT2 = resistance at temperature T2
  • RT1 = resistance at temperature T1
  • a  = temperature coefficient of resistance
  • T = temperature difference between T1 and T2

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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