A platinum resistance thermometer (PRT) is a piece of platinum wire which determines the temperature by measuring its electrical resistance. It is referred to as a temperature sensor. When manufactured carefully, these devices offer an excellent combination of sensitivity, range and reproducibility.

PRTs work in the following way. The electrical resistance of many metals (eg copper, silver, aluminium, platinum) increases approximately linearly with absolute temperature and this feature makes them useful as temperature sensors. The resistance of a wire of the material is measured by passing a current through it and measuring the voltage across it with a suitable voltmeter. The reading is converted to temperature using a calibration equation.

The most reproducible type of sensor is made from platinum because it is a stable unreactive metal which can be drawn down to fine wires but is not too soft. Using very pure wires, thermometers can be made with closely similar resistance characteristics and achieve good reproducibility in use.

The length and diameter of the platinum wire used in a thermometer are often chosen so that the resistance of the device at around 0 ºC is 100 ohms. Such a sensor is a called a PT100 sensor, and its resistance changes by approximately 0.4 ohms per ºC.

These are used to accurately measure the temperature in nuclear reactors.