Editing Heating element (section)

=== Resistance & resistivity ===
[[File:Resistivity_geometry.png|thumb|A piece of resistive material with electrical contacts on both ends]]
Materials used in heating elements have a relatively high [[Electrical resistivity and conductivity|electrical resistivity]], which is a measure of the material's ability to resist electric current. The [[Electrical resistance and conductance|electrical resistance]] that some amount of element material will have is defined by ''Pouillet's law'' as <math display="block">R = \rho \frac{\ell}{A}</math>where

* <math>R</math> is the electrical resistance of a uniform specimen of the material
* <math>\rho</math> is the resistivity of the material
* <math>\ell</math> is the [[length]] of the specimen
* <math>A</math> is the [[Cross section (geometry)|cross-sectional area]] of the specimen

The ''resistance per wire length'' (Ω/m) of a heating element material is defined in ASTM and DIN standards.<ref name=":3" />{{Rp|page=2}}<ref name=":4" /><ref name=":5" /> In ASTM, wires greater than 0.127&nbsp;mm in diameter are specified to be held within a tolerance of ±5% Ω/m and for thinner wires ±8% Ω/m.