Editing Thermistor (section)

==Basic operation==
Assuming, as a first-order approximation, that the relationship between resistance and temperature is [[linear]], then
: <math>\Delta R = k\,\Delta T,</math>
where
: <math>\Delta R</math>, change in resistance,
: <math>\Delta T</math>, change in temperature,
: <math>k</math>, first-order [[Temperature coefficient|temperature coefficient of resistance]].

Depending on type of the thermistor in question the <math>k</math> may be either positive or negative.

If <math>k</math> is [[Positive number|positive]], the resistance increases with increasing temperature, and the device is called a [[Temperature coefficient#Positive temperature coefficient of resistance|''positive-temperature-coefficient'']] (''PTC'') ''thermistor'', or ''posistor''. There are two types of PTC resistor{{dash}}''switching thermistor'' and ''silistor''. If <math>k</math> is negative, the resistance decreases with increasing temperature, and the device is called a [[Temperature coefficient#Negative temperature coefficient|''negative-temperature-coefficient'']] (''NTC'') ''thermistor''. Resistors that are not thermistors are designed to have a <math>k</math>  as close to 0 as possible so that their resistance remains nearly constant over a wide temperature range.

Instead of the temperature coefficient ''k'', sometimes the ''temperature coefficient of resistance'' <math>\alpha_T</math> ("alpha sub T") is used. It is defined as<ref name="Thermistor Terminology">[https://www.littelfuse.com/technical-resources/technical-centers/temperature-sensors/thermistor-info/thermistor-terminology.aspx Thermistor Terminology]. Littlefuse Technical Resources.</ref>
: <math>\alpha_T = \frac{1}{R(T)} \frac{dR}{dT}.</math>
This <math>\alpha_T</math> coefficient should not be confused with the <math>a</math> parameter below.