A thermometer is a temperature measuring device. Most methods use the temperature dependence of liquids and gases and determine the temperature from the coefficient of expansion. Corresponding methods are used in liquid thermometers. Other methods make use of the resistance changes of hot conductors and cold conductors or those of metals and alloys.
In addition to the method used, thermometers also differ in the type of temperature display, which can be analog or digital. Analog indicating thermometers are usually liquid thermometers, where the expanded liquid is fed into a thin glass tube attached to a scale. It is different with metallic detectors, whose resistance increases with temperature. In this method, which works with RTD elements, Resistance Temperature Detector (RTD), the change in resistance is detected via a current measurement digitized in an AD converter and displayed on a digital display. There are also thermal sensors based on semiconductors that generate an electrical voltage in response to temperature changes by means of the Seebeck effect.
In addition to the above-mentioned methods, there are also thermometers that determine the temperature by means of the emitted wavelength. These are pyrometers. In addition to the classic direct-reading thermometers, there are also radio thermometers in which the thermosensor can be positioned at some distance from the receiving display.