The Italian scientist Galileo Galilei invented the first thermometer in 1593. His instrument, which he called a thermoscope, consisted of an air-filled glass bulb with a narrow glass tube, the end of which was open and immersed in a container of colored water. Variations in the temperature of the air in the bulb caused the water to rise or fall in the tube, on which a scale was marked.

In later thermometers, Galileo's arrangement was reversed; the bulb was filled with a liquid, usually water or alcohol, and the tube was open to the air. The first sealed thermometers, using alcohol, were developed about 1641 by Ferdinand II, the Grand Duke of Tuscany. The mercury thermometer was invented in 1714 by the German physicist Gabriel Fahrenheit.

The scale developed by the French physicist Ren Antoine Raumur in 1731 was the first to use the freezing point of water as zero. The Swedish mathematician Anders Celsius in 1742 proposed the 100-unit scale. Research in thermodynamics led to the Rankine scale (named in honor of the Scottish engineer and physicist William J. M. Rankine) and the scale developed by William Thomson, later Baron Kelvin.