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Thermometers

A thermometer is a device which measures temperature or temperature gradient, using a variety of different principles. The word thermometer is derived from two smaller word fragments: thermo from the Latin (or Greek) for heat and meter from Latin, meaning to measure. more...

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Thermometers can be divided into two groups according to the level of knowledge about the physical basis of the underlying thermodynamic laws and quantities. For primary thermometers the measured property of matter is known so well that temperature can be calculated without any unknown quantities. Examples of these are thermometers based on the equation of state of a gas, on the velocity of sound in a gas, on the thermal noise voltage or current of an electrical resistor, and on the angular anisotropy of gamma ray emission of certain radioactive nuclei in a magnetic field.

Secondary thermometers are most widely used because of their convenience. Also, they are often much more sensitive than primary ones. For secondary thermometers knowledge of the measured property is not sufficient to allow direct calculation of temperature. They have to be calibrated against a primary thermometer at least at one temperature or at a number of fixed temperatures. Such fixed points, for example, triple points and superconducting transitions, occur reproducibly at the same temperature.

Internationally agreed temperature scales are based on fixed points and interpolating thermometers. The most recent official temperature scale is ITS-90 from 1990. It extends from 0.65 to above 1200 Kelvins.

Early History

The first thermometer was a thermoscope. Different versions of the thermoscope were invented by several inventors around the same time. The first to put a numerical scale on the thermoscopes was the Italian inventor Santorio Santorio for use in medicine. In 1593, Galileo Galilei invented a rudimentary water thermometer (using the contraction of air to draw water up a tube). He also discovered that liquids of lesser density than water could be suspended within it and that they would float at different heights depending on the temperature; hence Galileo's Thermoscope--Galileo's use of alcohol enclosed in glass spheres floating in a column of water in order to measure their differences in temperature. In 1714, Daniel Gabriel Fahrenheit invented the first mercury thermometer.

This history of the thermometer, from its invention in the early sixteenth century (an achievement attributed to at least four scientists, including Galileo) through various changes and applications over the next three centuries, includes controversy about its invention, the story of different scales, from Fahrenheit and Celsius to the now-forgotten Reaumur, Delisle, and Christin scales, and the history of the gradual scientific then popular understanding of the concept of temperature. Not until 1800 did people interested in thermometers begin to see clearly what they were measuring, and the impetus for improving thermometry came largely from study of the weather -- the liquid-in-glass thermometer became the meteorologist's instrument before that of the chemist or physicist.