Thermometer

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A clinical mercury thermometer
The thermometer is a device that measures temperature or temperature gradient using a variety of different principles; it comes from the Greek roots thermo, heat, and meter, to measure. A thermometer has two important elements: the temperature sensor (e.g. the bulb on a mercury thermometer) in which some physical change occurs with temperature, plus some means of converting this physical change into a value (e.g. the scale on a mercury thermometer). Industrial thermometers commonly use electronic means to provide a digital display or input to a computer.
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 (see Johnson鏈測quist 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 the International Temperature Scale of 1990. It extends from 0.65K (?272.5鐧�; ?458.5鐧�) to approximately 1,358K (1,085鐧�; 1,985鐧�).
Contents
1 Early history
2 Types of thermometers
3 Calibration
4 Special uses of thermometers
5 See also
6 Notes
7 References
8 Further reading
9 External links
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Early history

Galileo thermometer
Various authors have credited the invention of the thermometer to Avicenna, Cornelius Drebbel, Robert Fludd, Galileo Galilei or Santorio Santorio. The thermometer was not a single invention, however, but a development.
Philo and Hero of Alexandria knew of the principle that certain substances, notably air, expand and contract and described a demonstration in which a closed tube partially filled with air had its end in a container of water.[1] The expansion and contraction of the air caused the position of the water/air interface to move along the tube.
Such a mechanism was later used to show the hotness and coldness of the air with a tube in which the water level is controlled by the expansion and contraction of the air. These devices were developed by Avicenna in the early 11th century,[2][3] and by several European scientists in the 16th and 17th centuries, notably Galileo Galilei. As a result, devices were shown to produce this effect reliably, and the term thermoscope (Galileo thermometer) was adopted because it reflected the changes in sensible heat (the concept of temperature was yet to arise). The difference between a thermoscope and a thermometer is that the latter has a scale.[4] Though Avicenna or Galileo are often said to be the inventor of the thermometer, what they produced were thermoscopes.
Galileo also discovered that objects (glass spheres filled with aqueous alcohol) of slightly different densities would rise and fall, which is nowadays the principle of the Galileo thermometer (shown). Today such thermometers are calibrated to a temperature scale.
The first clear diagram of a thermoscope was published in 1617 by Giuseppe Biancani: the first showing a scale and thus constituting a thermometer was by Robert Fludd in 1638. This was a vertical tube, with a bulb at the top and the end immersed in water. The water level in the tube is controlled by the expansion and contraction of the air, so it is what we would now call an air thermometer.[5]
The first person to put a scale on a thermoscope is variously said to be Francesco Sagredo[6] or Santorio Santorio[7] in about 1611 to 1613.
The word thermometer (in its French form) first appeared in 1624 in La R闁弐闁峵ion Math闁檃tique by J. Leurechon, who describes one with a scale of 8 degrees.[8]
The above instruments suffered from the disadvantage that they were also barometers, i.e. sensitive to air pressure. In about 1654 Ferdinando II de' Medici, Grand Duke of Tuscany, made sealed tubes part filled with alcohol, with a bulb and stem, the first modern-style thermometer, depending on the expansion of a liquid, and...(and so on)

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