Editing Pyrometer (section)

== History ==
[[File:Pyrometer example.png|thumb|left|A pyrometer from 1852. Heating the metal bar (a) presses against a lever (b), which  moves a pointer (c) along a scale that serves as a measuring index. (e) is an immovable prop which holds the bar in place. A spring on (c) pushes against (b), causing the index to fall back once the bar cools.]]

The term "pyrometer" was coined in the 1730s by [[Pieter van Musschenbroek]], better known as the inventor of the [[Leyden jar]]. His device, of which no surviving specimens are known, may be now called a dilatometer because it measured the dilation of a metal rod.<ref>{{cite news |url=https://catalogue.museogalileo.it/object/PyrometerDilatometer.html |title=Museo Galileo - Pyrometer or dilatometer }}</ref>

The earliest example of a pyrometer thought to be in existence is the [https://collection.sciencemuseumgroup.org.uk/objects/co1668/hindleys-pyrometer-pyrometers-dilatometers Hindley Pyrometer] held by the London [[Science Museum, London|Science Museum]], dating from 1752, produced for the Royal collection. The pyrometer was a well known enough instrument that it was described in some detail by the mathematician [[Leonhard Euler|Euler]] in 1760.<ref>{{Cite book |last=Euler |first=Leonhard |title=Letters of Euler on Different Subjects in Physics and Philosophy, Addressed to a German Princess. With Notes, and a Life of Euler |year=1823 |language=en |translator=Henry Hunter}}</ref>

Around 1782 potter [[Josiah Wedgwood]] invented a different type of pyrometer (or rather a [[pyrometric device]]) to measure the temperature in his kilns,<ref name=jw1>{{cite web |url=https://www.bbc.co.uk/history/historic_figures/wedgwood_josiah.shtml |title=History&nbsp;— Historic Figures: Josiah Wedgwood (1730–1795) |publisher=[[BBC]] |date=1970-01-01 |access-date=2013-08-31}}</ref> which first compared the color of clay fired at known temperatures, but was eventually upgraded to measuring the shrinkage of pieces of clay, which depended on kiln temperature (see [[Wedgwood scale]] for details).<ref name=wm1>{{cite web |url=http://www.wedgwoodmuseum.org.uk/learning/discovery-packs/pack/lives-of-the-wedgwoods/chapter/pyrometer |title=Pyrometer |publisher=[[Wedgwood Museum]] |access-date=23 August 2013}}</ref> Later examples used the expansion of a metal bar.<ref>{{cite book |last=Draper |first=John William |title=A Textbook on chemistry |date=1861 |publisher=Harper & Bros |page=[https://archive.org/details/bub_gb_HKwS7QDh5eMC/page/n38 24] |url=https://archive.org/details/bub_gb_HKwS7QDh5eMC |quote=draper, john william. }}</ref>

In the 1860s–1870s brothers William and [[Werner Siemens]] developed a platinum [[resistance thermometer]], initially to measure temperature in undersea cables, but then adapted for measuring temperatures in metallurgy up to 1000&nbsp;°C, hence deserving a name of a pyrometer.

Around 1890 [[Henry Louis Le Chatelier]] developed the [[thermoelectric]] pyrometer.<ref name="frs">{{Cite journal | last1 = Desch | first1 = C. H. | title = Robert Abbott Hadfield. 1858–1940 | doi = 10.1098/rsbm.1941.0027 | journal = [[Obituary Notices of Fellows of the Royal Society]] | volume = 3 | issue = 10 | pages = 647–664| year = 1941 | s2cid = 178057481 | doi-access = free }}</ref>

[[File:Silicon grown by Czochralski process 1956.jpg|thumb|upright|Technician measuring the temperature of molten [[silicon]] at {{convert|2650|°F}} with a [[disappearing-filament pyrometer]] in [[Czochralski process|Czochralski]] crystal growing equipment at Raytheon transistor plant in 1956]]

The first [[disappearing-filament pyrometer]] was built by L.&nbsp;Holborn and F.&nbsp;Kurlbaum in 1901.<ref name=Michalski>{{cite book |last1=Michalski |first1=L. |last2=Eckersdorf |first2=K. |last3=Kucharski |first3=J. |last4=McGhee |first4=J. |title=Temperature Measurement |date=2001 |publisher=John Wiley & Sons |isbn=978-0-471-86779-1 |pages=162–208 }}</ref> This device had a thin electrical filament between an observer's eye and an incandescent object. The current through the filament was adjusted until it was of the same colour (and hence temperature) as the object, and no longer visible; it was calibrated to allow temperature to be inferred from the current.<ref name=Mercer>{{cite book |last1=Mercer |first1=Carolyn |title=Optical Metrology for Fluids, Combustion and Solids |date=2003 |publisher=Springer Science & Business Media |isbn=978-1-4020-7407-3 |pages=297–305 }}</ref>

The temperature returned by the vanishing-filament pyrometer and others of its kind, called brightness pyrometers, is dependent on the [[emissivity]] of the object. With greater use of brightness pyrometers, it became obvious that problems existed with relying on knowledge of the value of emissivity. Emissivity was found to change, often drastically, with surface roughness, bulk and surface composition, and even the temperature itself.<ref name=Ng>{{cite journal |last1=Ng |first1=Daniel |last2=Fralick |first2=Gustave |title=Use of a multiwavelength pyrometer in several elevated temperature aerospace applications |journal=Review of Scientific Instruments |date=2001 |volume=72 |issue=2 |pages=1522 |doi=10.1063/1.1340558 |bibcode=2001RScI...72.1522N |hdl=2060/20010035857 |s2cid=52218391 |hdl-access=free }}</ref>

To get around these difficulties, the ''ratio'' or ''two-color'' pyrometer was developed. They rely on the fact that [[Planck's law]], which relates temperature to the intensity of radiation emitted at individual wavelengths, can be solved for temperature if Planck's statement of the intensities at two different wavelengths is divided. This solution assumes that the emissivity is the same at both wavelengths<ref name=Mercer/> and cancels out in the division. This is known as the [[emissivity#Explanation|gray-body assumption]]. Ratio pyrometers are essentially two brightness pyrometers in a single instrument. The operational principles of the ratio pyrometers were developed in the 1920s and 1930s, and they were commercially available in 1939.<ref name=Michalski/>

As the ratio pyrometer came into popular use, it was determined that many materials, of which metals are an example, do not have the same emissivity at two wavelengths.<ref name = Olinger>{{cite conference |url=http://pyrometry.com/farassociates_icipaper.pdf |author=D. Olinger |author2=J. Gray |author3=R. Felice |title=Successful Pyrometry in Investment Casting |conference=Investment Casting Institute 55th Technical Conference and Expo |publisher=Investment Casting Institute |date=2007-10-14 |access-date=2015-04-02}}</ref> For these materials, the emissivity does not cancel out, and the temperature measurement is in error. The amount of error depends on the emissivities and the wavelengths where the measurements are taken.<ref name=Mercer/> Two-color ratio pyrometers cannot measure whether a material's emissivity is wavelength-dependent.

To more accurately measure the temperature of real objects with unknown or changing emissivities, multiwavelength pyrometers were envisioned at the US [[National Institute of Standards and Technology]] and described in 1992.<ref name=Michalski/> Multiwavelength pyrometers use three or more wavelengths and mathematical manipulation of the results to attempt to achieve accurate temperature measurement even when the emissivity is unknown, changing or differs according to wavelength of measurement.<ref name=Mercer/><ref name= Ng/><ref name = Olinger/>