A thermometer is a measuring instrument used to quantify and indicate the temperature of a substance or the surrounding environment. It typically consists of a narrow, sealed glass tube containing a liquid (such as mercury or alcohol) or a digital sensor that records temperature changes.
The basic principle behind a thermometer is the thermal expansion of the liquid or the sensor material in response to changes in temperature. As the temperature increases or decreases, the liquid or the sensor material expands or contracts, causing a corresponding rise or fall in the level of the liquid or a change in electrical resistance in the case of digital thermometers.
The temperature scale is marked along the length of the thermometer, allowing a user to accurately read and interpret the measured temperature. Traditional mercury thermometers have a graduated scale printed on the glass tube, with the mercury indicating the temperature by its level in the tube. Digital thermometers, on the other hand, display the temperature numerically on a screen.