What does a thermometer measure?

A thermometer measures temperature. Temperature describes how hot or cold something is, while heat refers to energy moving between objects because of a temperature difference.

Traditional liquid-in-glass thermometers use a material such as mercury or colored alcohol. As the liquid warms, it expands and rises in a narrow tube; as it cools, it contracts and falls. The position is compared with a calibrated scale, commonly Celsius or Fahrenheit. Digital thermometers use electronic sensors, while infrared models detect thermal radiation without touching the object.

The first devices for comparing temperatures, called thermoscopes, appeared in Europe during the 1600s. Daniel Gabriel Fahrenheit developed a reliable mercury thermometer in the early 1700s, and Anders Celsius proposed the Celsius scale in 1742. A thermometer measures temperature—not distance, height, speed, or the total amount of heat in an object.