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 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

where

• 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.