a thermistor
/ THUR-mis-ter /
A thermistor is a resistor that takes its temperature seriously. An ordinary resistor tries hard to keep the same resistance no matter how warm it gets; a thermistor does the opposite on purpose — its resistance changes a lot with temperature, and that is the whole point. Warm it up and (for the common NTC type) its resistance drops sharply. Read the resistance and you have read the temperature.
Most thermistors are NTC, meaning Negative Temperature Coefficient: hotter means lower resistance. They are made from metal-oxide ceramics and are very sensitive — a few percent change in resistance per degree near room temperature, far more than a metal sensor gives. The catch is that the relationship is strongly curved, not a straight line. Engineers describe it with the Beta equation or the more accurate Steinhart-Hart equation. A typical part is specified as 10 kΩ at 25 °C with a Beta of about 3950 K; you put it in a voltage divider, measure the voltage, work back to resistance, then use the equation (or a lookup table) to get temperature. There is also a less common PTC type whose resistance rises with heat, often used for protection rather than precise measurement.
Why this matters: thermistors are cheap, tiny, fast, and very sensitive, so they are everywhere — battery packs, 3D-printer hot ends, thermostats, medical thermometers. The honest caveats: their curve must be linearized in software or by clever resistor tricks; the part-to-part spread can be a degree or more unless you buy graded parts or calibrate; and because you measure temperature by passing current through the device, too much current heats the thermistor itself (self-heating) and you end up measuring your own measurement. Keep the sense current small.
A 10 kΩ NTC thermistor sits in the bottom leg of a divider with a 10 kΩ fixed resistor on 3.3 V. At 25 °C both are 10 kΩ, so the tap reads 1.65 V. Heat the thermistor to about 50 °C and its resistance falls to roughly 4 kΩ, dropping the tap to about 0.94 V — the microcontroller turns that voltage back into degrees.
Resistance falls with heat; a divider turns that into a voltage to read.
A thermistor's response is steeply nonlinear, so a single linear formula will be wrong over a wide range — always linearize with the Beta or Steinhart-Hart equation or a table. And keep the sense current tiny, or self-heating adds its own error.