Editing Linear approximation (section)

===Electrical resistivity===
{{main|Electrical resistivity}}
The electrical resistivity of most materials changes with temperature. If the temperature ''T'' does not vary too much, a linear approximation is typically used:
<math display="block">\rho(T) = \rho_0[1+\alpha (T - T_0)]</math>
where <math>\alpha</math> is called the ''temperature coefficient of resistivity'', <math>T_0</math> is a fixed reference temperature (usually room temperature), and <math>\rho_0</math> is the resistivity at temperature <math>T_0</math>. The parameter <math>\alpha</math> is an empirical parameter fitted from measurement data. Because the linear approximation is only an approximation, <math>\alpha</math> is different for different reference temperatures. For this reason it is usual to specify the temperature that <math>\alpha</math> was measured at with a suffix, such as <math>\alpha_{15}</math>, and the relationship only holds in a range of temperatures around the reference.<ref>{{cite book |first=M. R. |last=Ward |year=1971 |title=Electrical Engineering Science |pages=36–40 |publisher=McGraw-Hill |isbn=0-07-094255-2 }}</ref> When the temperature varies over a large temperature range, the linear approximation is inadequate and a more detailed analysis and understanding should be used.