Thermometer
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{{Mergefrom|Medical thermometer|Talk:Thermometer#Merge proposal with medical thermometer|date=January 2008}}
[[Image:Clinical thermometer 38.7.JPG|thumb|right|A clinical [[mercury thermometer]]]]
The '''thermometer''' is a device that measures [[temperature]] or [[temperature gradient]] using a variety of different principles. The word thermometer is derived from two smaller word fragments: ''thermo'' from the [[Greek language|Greek]] for [[heat]] and ''meter'' also from Greek, meaning to measure. A thermometer has two important elements: the temperature sensor (e.g. the bulb on a [[mercury (element)|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 [[thermodynamics|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–Nyquist noise]]) [[voltage]] or [[electric current|current]] of an electrical resistor, and on the angular [[anisotropy]] of [[gamma ray]] emission of certain [[radioactive decay|radioactive]] [[atomic nucleus|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 point]]s and [[Superconductivity|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 0.65 [[kelvin|K]] to approximately 1358 K (−272.5 °[[Celsius|C]] to 1085 °C).
== Early history ==
[[Image:Galileo Thermometer closeup.jpg|thumb|right|150px|[[Galileo thermometer]]]]
Various authors have credited the invention of the thermometer to [[Avicenna|Abū Alī ibn Sīnā]] (usually known as Avicenna), [[Cornelius Drebbel]], [[Robert Fludd]], [[Galileo Galilei]] or [[Sanctorius|Santorio Santorio]]. But the thermometer was not a single invention, it was 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.<ref>T. D. McGee (1988) ''Principles and Methods of Temperature Measurement'' ISBN 0471627674</ref> 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 air thermometers were developed by [[Avicenna]] in the early 11th century,<ref>[[Robert Briffault]] (1938). ''The Making of Humanity'', p. 191</ref><ref>Fatima Agha Al-Hayani (2005). "Islam and Science: Contradiction or Concordance", ''Zygon'' '''40''' (3), p. 565-576.</ref> 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]] 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<ref>T. D. McGee (1988) ''Principles and Methods of Temperature Measurement'' page 3, ISBN 0471627674</ref>. Though Galileo is often said to be the inventor of the thermometer, what he produced was a thermoscope.
[[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.<ref>T. D. McGee (1988) ''Principles and Methods of Temperature Measurement'', pages 2-4 ISBN 0471627674</ref>
The first person to put a scale on a thermoscope is variously said to be [[Francesco Sagredo]]<ref>J. E. Drinkwater (1832)''Life of Galileo Galilei'' page 41</ref> or [[Sanctorius|Santorio Santorio]]<ref>[http://galileo.rice.edu/sci/santorio.html The Galileo Project: Santorio Santorio]</ref> in about [[1611]] to [[1613]].
The word thermometer (in its French form) first appeared in [[1624]] in ''La Récréation Mathématique'' by J. Leurechon, who describes one with a scale of 8 degrees<ref>R. P. Benedict (1984) Fundamentals of Temperature, Pressure, and Flow Measurements, 3rd ed, ISBN 0-471-89383-8 page 4</ref>.
The above instruments suffered from the disadvantage that they were also [[barometer]]s, 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 independent of air pressure<ref>R. P. Benedict (1984) Fundamentals of Temperature, Pressure, and Flow Measurements, 3rd ed, ISBN 0-471-89383-8 page 4</ref>. Many other scientists experimented with various liquids and designs of thermometer.
However, each inventor and each thermometer was unique—there was no standard scale. In [[1665]] [[Christian Huygens]] suggested using the melting and boiling points of water as standards, and in [[1694]] [[Carlo Renaldini]] proposed using them as fixed points on a universal scale. In [[1701]] [[Isaac Newton]] proposed a scale of 12 degrees between the melting point of ice and [[body temperature]]. Finally in [[1724]] [[Daniel Gabriel Fahrenheit]] produced a temperature scale which now (slightly adjusted) bears his name. He could do this because he manufactured thermometers, using [[mercury (element)|mercury]] (which has a high coefficient of expansion) for the first time and the quality of his production could provide a finer scale and greater reproducibility, leading to its general adoption. In [[1742]] [[Anders Celsius]] proposed a scale with zero at the boiling point and 100 degrees at the melting point of water<ref>R. P. Benedict (1984) Fundamentals of Temperature, Pressure, and Flow Measurements, 3rd ed, ISBN 0-471-89383-8 page 6</ref>, though the scale which now bears his name has them the other way around
In [[1866]] Sir [[Thomas Clifford Allbutt]] invented a [[Medical thermometer|clinical thermometer]] that produced a body temperature reading in five minutes as opposed to twenty<ref name="Allbutt">[http://www.britannica.com/eb/article-9005775 Sir Thomas Clifford Allbutt], ''[[Encyclopædia Britannica]]''</ref>.
== Types of thermometers ==
[[Image:Maximum thermometer close up 2.JPG|thumb|right|[[Mercury-in-glass thermometer]]]]
[[Image:Thermometers in pitcher.jpg|thumb|right|Cooking thermometers used to measure the temperature of steamed milk]]
Thermometers have been built which utilise a range of physical effects to measure temperature. Most thermometers are originally calibrated to a constant-volume [[gas thermometer]].{{Fact|date=January 2008}} Temperature sensors are used in a wide variety of scientific and engineering applications, especially measurement systems. Temperature systems are primarily either electrical or mechanical, occasionally inseparable from the system which they control (as in the case of a [[mercury thermometer]]).
*[[Alcohol thermometer]]
*[[Beckmann thermometer|Beckmann differential thermometer]]
*[[Bi-metallic strip|Bi-metal mechanical thermometer]]
*[[Electrical resistance thermometer]]
*[[Galileo thermometer]]
*[[Infrared thermometer]]
*[[Liquid Crystal Thermometer]]
*[[Medical thermometer]] (e.g. oral thermometer, rectal thermometer, basal thermometer)
*[[Mercury-in-glass thermometer]]
*[[Reversing thermometer]]
*[[Silicon bandgap temperature sensor]]
*[[Six's thermometer]]- also known as a '''Maximum minimum thermometer'''
*[[Thermistor]]
*[[Thermocouple]]
*[[Coulomb blockade|Coulomb blockade thermometer]]
*[[Pill Thermometer]]
==Calibration==
Thermometers can be calibrated either by comparing them with other certified thermometers or by checking them against known fixed points on the temperature scale. The best known of these fixed points are the melting and boiling points of pure water. (Note that the boiling point of water varies with pressure, so this must be controlled.)
The traditional method of putting a scale on a liquid-in glass or or liquid-in-metal thermometer was in three stages:
* 1. Immerse the sensing portion in a stirred mixture of pure ice and water and mark the point indicated when it had come to thermal equilibrium.
* 2. Immerse the sensing portion in a steam bath at one standard atmosphere (101.325 kPa = 760 mm Hg) and again mark the point indicated.
* 3. Divide the distance between these marks into equal portions according to the temperature scale being used.
Other fixed points were used in the past are the [[body temperature]] (of a healthy adult male) which was originally used by [[Fahrenheit]] as his upper fixed point (96 degrees F to be a number divisible by 12) and the lowest temperature given by a mixture of salt and ice, which was originally the definition of 0 degrees [[Fahrenheit]]<ref>R. P. Benedict (1984) ''Fundamentals of Temperature, Pressure, and Flow Measurements'', 3rd ed, ISBN 0-471-89383-8, page 5 </ref>. (This is an example of a [[Frigorific mixture]]). As body temperature varies, the [[Fahrenheit]] scale was later changed to use an upper fixed point of boiling water at 212 degrees<ref>J. Lord (1994) ''Sizes'' ISBN 0 06 273228 5 page 293</ref>.
These have now been replaced by the defining points in the [[International Temperature Scale of 1990]], though in practice the melting point of water is more commonly used than its triple point, the latter being more difficult to manage and thus restricted to critical standard measurement. Nowadays manufacturers will often use a [[thermostat]] bath or solid block where the temperature is held constant relative to a calibrated thermometer. Other thermometers to be calibrated are put into the same bath or block and allowed to come to equilibrium, then the scale marked, or any deviation from the instrument scale recorded<ref>R. P. Benedict (1984) ''Fundamentals of Temperature, Pressure, and Flow Measurements'', 3rd ed, ISBN 0-471-89383-8, chapter 11 "Calibration of Temperature Sensors" </ref>. For many modern devices calibration will be stating some value to be used in processing an electronic signal to convert it to a temperature .
== Specialist uses of thermometers ==
*[[Candy thermometer]]
*[[Meat thermometer]]
*[[Medical thermometer]]
== See also ==
*[[Automated airport weather station]]
*[[Timeline of temperature and pressure measurement technology]]
*[[Comparison of temperature scales]]
*[[Resistance Thermometer]]
*[[Thermistors]]
*[[Thermocouples]]
*[[Thermogenerator]]
*[[Silicon bandgap temperature sensor]]
==Notes==
{{reflist}}
==References==
* [http://www.history.com/encyclopedia.do?articleId=212819 History Channel - Invention] - Notable Modern Inventions and Discoveries
* [http://inventors.about.com/library/inventors/blthermometer.htm About - Thermometer] - Thermometers - Early History, Anders Celsius, Gabriel Fahrenheit and Thomson Kelvin.
* [http://www.syvum.com/cgi/online/serve.cgi/squizzes/physics/thermometers.html Thermometers and Thermometric Liquids] - Mercury and Alcohol.
* [http://www.cstl.nist.gov/div836/836.05/papers/Vaughn01TM_IT_Cal_Lab.pdf The NIST Industrial Thermometer Calibration Laboratory]
==Further reading==
* Middleton, W. E. K. (1966). ''A history of the thermometer and its use in meteorology''. Baltimore: Johns Hopkins Press. Reprinted ed. 2002, ISBN 0801871530.
==External links==
{{Wiktionary}}
{{commonscat|Measuring instruments (temperature)}}
* [http://www.zytemp.com/tutorial/History_Of_Thermometry.htm History of Temperature and Thermometry]
* [http://chemeducator.org/sbibs/s0005002/spapers/520088jw.htm ''The Chemical Educator'', Vol. 5, No. 2 (2000)] The Thermometer—From The Feeling To The Instrument
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