The fist 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 most thermometer work is that a liquid or gas used for measuring expands or contracts with changes in temperature more rapidly than the glass containing it. Thus when a coloured 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 scientists 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 320 is the freezing point of water and 2120 the boiling point. Mercury is still used in most thermometers because it has a high boiling point (6740) and a low freezing point (-380).
An alcohol thermometer, still in use in some countries, was made by Rene de Reaumur, a French naturalist, about 1731. About 11 years later Andres Celsius, a Swedish astronomer, used the centigrade scale for the first time, with freezing point 00 and boiling point at 1000.
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