A thermometer is a device that measures temperature or a temperature gradient. A thermometer has two important elements: the temperature sensor (e.g. the bulb on a mercury-in-glass thermometer) in which some physical change occurs with temperature, plus some means of converting this physical change into a numerical value (e.g. the visible scale that is marked on a mercury-in-glass thermometer).
There are various principles by which different thermometers operate. They include the thermal expansion of solids or liquids with temperature, or the change in pressure of a gas on heating or cooling. Radiation-type thermometers measure the infrared energy emitted by an object, allowing measurement of temperature without contact.
Thermometers are widely used in industry to control and regulate processes, in the study of weather, in medicine, and scientific research.