The first practical thermometer or instrument for measuring temperature was invented shortly before the end of the 16th century by the famous Italian astronomer Galileo. It was an air thermometer giving only a rough indication of the degrees of heat and cold, and later he increased its efficiency by using alcohol instead of air.
The principle on which thermometers work is that the liquid or gas used for measuring expands or contracts with changes in temperature more rapidly than the glass containing it. Thus when a colored liquid is confined in a thin glass tube the difference in expansion, as shown by the level of the liquid against a graduated scale, indicates the temperature.
About 1714 the German scientist Gabriel Daniel Fahrenheit designed a thermometer which, for the first time, used mercury as the measuring agent. He also introduced the scale named after him in which 32° is the freezing point of water and 212° the boiling point. Mercury is still used in most thermometers because if has a high boiling point (674°) and a low freezing point (-38°).
An alcohol thermometer, still in use in some countries was made by René de Réaumur, a French naturalist, about 1731. About 11 years later Anders Celsius, a Swedish astronomer used the centigrade scale for the first time, with freezing point at 0°and boiling point at 100°.