Editing Magnetometer (section)

==Introduction==

===Magnetic fields===
Magnetic fields are [[vector (mathematics)|vector]] quantities characterized by both strength and direction. The strength of a magnetic field is measured in units of [[tesla (unit)|tesla]] in the [[SI unit]]s, and in [[gauss (unit)|gauss]] in the [[cgs system]] of units. 10,000 gauss are equal to one tesla.<ref name=Macintyre2000>{{cite web|last=Macintyre|first=Steven A.|title=Magnetic field measurement.|url=http://engineering.dartmouth.edu/dartmag/docs/macintyre.pdf|work=ENG Net Base (2000)|publisher=CRC Press LLC|access-date=29 March 2014|archive-date=19 March 2015|archive-url=https://web.archive.org/web/20150319010220/http://engineering.dartmouth.edu/dartmag/docs/macintyre.pdf|url-status=dead}}</ref> Measurements of the Earth's magnetic field are often quoted in units of nanotesla (nT), also called a gamma.<ref name="FS–236–95">{{cite web|title=USGS FS–236–95: Introduction to Potential Fields: Magnetics|url=http://pubs.usgs.gov/fs/fs-0236-95/fs-236-95.pdf|publisher=USGS|access-date=29 March 2014}}</ref> The Earth's magnetic field can vary from 20,000 to 80,000 nT depending on location, fluctuations in the Earth's magnetic field are on the order of 100 nT, and magnetic field variations due to [[magnetic anomaly|magnetic anomalies]] can be in the picotesla (pT) range.<ref name=Hovde2013>{{cite book|author1=D. C. Hovde|author2=M. D. Prouty|author3=I. Hrvoic|author4=R. E. Slocum|title="Commercial magnetometers and their application", in the book "Optical Magnetometry"|date=2013|publisher=Cambridge University Press|isbn=978-0-511-84638-0|pages=387–405|url=http://ebooks.cambridge.org/chapter.jsf?bid=CBO9780511846380&cid=CBO9780511846380A137&tabName=Chapter|access-date=30 March 2014|archive-date=7 April 2014|archive-url=https://web.archive.org/web/20140407080700/http://ebooks.cambridge.org/chapter.jsf?bid=CBO9780511846380&cid=CBO9780511846380A137&tabName=Chapter|url-status=dead}}</ref> ''Gaussmeters'' and ''teslameters'' are magnetometers that measure in units of gauss or tesla, respectively. In some contexts, magnetometer is the term used for an instrument that measures fields of less than 1 millitesla (mT) and gaussmeter is used for those measuring greater than 1 mT.<ref name=Macintyre2000/>

===Types of magnetometer===
[[File:Juno mag boom3.jpg|thumb|The Magnetometer experiment for the [[Juno (spacecraft)|Juno]] orbiter for Juno can be seen here on the end of a boom. The spacecraft uses two fluxgate magnetometers. (see also [[Magnetometer (Juno)]])]]
There are two basic types of magnetometer measurement. ''Vector magnetometers'' measure the vector components of a magnetic field. ''Total field magnetometers'' or ''scalar magnetometers'' measure the magnitude of the vector magnetic field.<ref name=Edelstein2007>{{cite journal|last=Edelstein|first=Alan|title=Advances in magnetometry|journal=[[Journal of Physics: Condensed Matter]]|date=2007|volume=19|issue=16|pages=165217 (28pp)|doi=10.1088/0953-8984/19/16/165217|doi-access=free |bibcode=2007JPCM...19p5217E|s2cid=108531365|s2cid-access=free|id={{zenodo|1235730}} }}</ref> Magnetometers used to study the Earth's magnetic field may express the vector components of the field in terms of [[Magnetic declination|''declination'']] (the angle between the horizontal component of the field vector and true, or geographic, north) and the ''inclination'' (the angle between the field vector and the horizontal surface).<ref name=Tauxe2014>{{cite web|last1=Tauxe|first1=L.|last2=Banerjee|first2=S.K.|last3=Butler|first3=R.F.|last4=van der Voo|first4=R.|title=Essentials of Paleomagnetism: Third Web Edition 2014|url=http://earthref.org/MAGIC/books/Tauxe/Essentials/WebBook3ch2.html#x4-140002|publisher=Magnetics Information Consortium (MagIC)|access-date=30 March 2014}}</ref>

''Absolute magnetometers'' measure the absolute magnitude or vector magnetic field, using an internal calibration or known physical constants of the magnetic sensor.<ref name=IAGA1996>{{cite book|author1=JERZY JANKOWSKI|author2=CHRISTIAN SUCKSDORFF|name-list-style=amp|title=IAGA GUIDE FOR MAGNETIC MEASUREMENTS AND OISERVAIORY PRACTICE|date=1996|publisher=International Association of Geomagnetism and Aeronomy|location=Warsaw|isbn=978-0-9650686-2-8|page=51|url=http://iugg.org/IAGA/iaga_pages/pdf/IAGA-Guide-Observatories.pdf|url-status=dead|archive-url=https://web.archive.org/web/20160304080050/http://iugg.org/IAGA/iaga_pages/pdf/IAGA-Guide-Observatories.pdf|archive-date=4 March 2016}}</ref> ''Relative magnetometers'' measure magnitude or vector magnetic field relative to a fixed but uncalibrated baseline. Also called ''variometers'', relative magnetometers are used to measure variations in magnetic field.

Magnetometers may also be classified by their situation or intended use. ''Stationary magnetometers'' are installed to a fixed position and measurements are taken while the magnetometer is stationary.<ref name=Edelstein2007/> ''Portable'' or ''mobile magnetometers'' are meant to be used while in motion and may be manually carried or transported in a moving vehicle. ''Laboratory magnetometers'' are used to measure the magnetic field of materials placed within them and are typically stationary. ''Survey magnetometers'' are used to measure magnetic fields in [[geomagnetic]] surveys; they may be fixed base stations, as in the [[INTERMAGNET]] network, or mobile magnetometers used to scan a geographic region. An early adoption (in the 1950s) of airborne magnetometry by [[Inco]] prompted the discovery of [[Thompson, Manitoba]].<ref name="thompson60">{{cite book |last1=Thompson |first1=John Fairfield |last2=Beasley |first2=Norman |title=For the Years to Come: A Story of International Nickel of Canada |date=1960 |publisher=Longmans, Green & Co |location=Toronto}}</ref>

===Performance and capabilities===
The performance and capabilities of magnetometers are described through their technical specifications. Major specifications include<ref name=Macintyre2000/><ref name=Hovde2013/>
*''Sample rate'' is the number of readings given per second. The inverse is the ''cycle time'' in seconds per reading. Sample rate is important in mobile magnetometers; the sample rate and the vehicle speed determine the distance between measurements.
*''Bandwidth'' or ''bandpass'' characterizes how well a magnetometer tracks rapid changes in magnetic field. For magnetometers with no onboard [[signal processing]], bandwidth is determined by the [[Nyquist limit]] set by sample rate. Modern magnetometers may perform smoothing or averaging over sequential samples, achieving a lower noise in exchange for lower bandwidth.
*''Resolution'' is the smallest change in a magnetic field the magnetometer can resolve. A magnetometer should have a resolution a good deal smaller than the smallest change one wishes to observe. This includes [[quantization error]] which is caused by recording roundoff and truncation of digital expressions of the data.
*''Absolute error'' is the difference between the readings of a magnetometer true magnetic field.
*''Drift'' is the change in absolute error over time.
*''Thermal stability'' is the dependence of the measurement on temperature. It is given as a temperature coefficient in units of nT per degree Celsius.
*''Noise'' is the random fluctuations generated by the magnetometer sensor or electronics. Noise is given in units of <math>\rm{nT}/\sqrt{\rm{Hz}}</math>, where frequency component refers to the bandwidth.
*''Sensitivity'' is the larger of the noise or the resolution.
*''Heading error'' is the change in the measurement due to a change in orientation of the instrument in a constant magnetic field.
*The ''dead zone'' is the angular region of magnetometer orientation in which the instrument produces poor or no measurements. All optically pumped, proton-free precession, and Overhauser magnetometers experience some dead zone effects.
*''Gradient tolerance'' is the ability of a magnetometer to obtain a reliable measurement in the presence of a magnetic field [[gradient]]. In surveys of [[unexploded ordnance]] or landfills, gradients can be large.

===Early magnetometers===
[[File:Stanley compass 1.jpg|thumb|right|The compass is a simple type of magnetometer.]]

[[File:Coast and Geodetic Survey Magnetometer Plate XV Fig 1 WBClark 1897.jpg|thumb|right|Coast and Geodetic Survey Magnetometer No. 18]]

The [[compass]], consisting of a magnetized needle whose orientation changes in response to the ambient magnetic field, is a simple type of magnetometer, one that measures the direction of the field. The oscillation frequency of a magnetized needle is proportional to the square-root of the strength of the ambient magnetic field; so, for example, the oscillation frequency of the needle of a horizontally situated compass is proportional to the square-root of the horizontal intensity of the ambient field.{{citation needed|date=October 2020}}

In 1823 William Scoresby (1789-1857), an English explorer, scientist and clergyman, was deeply involved in magnetic science, particularly in improving ships' compasses.  In 1823, he published a paper in the ''Transactions of the Royal Society of Edinburgh'' titled "Description of Magnetimenter, being a new instrument for measuring magnetic attractions and finding the dip of the needle; with an accont of experiments made with it." 

In 1833, [[Carl Friedrich Gauss]], head of the Geomagnetic Observatory in Göttingen, published a paper on measurement of the Earth's magnetic field.<ref>{{cite web |author=Gauss, C.F. |date=1832 |title=The Intensity of the Earth's Magnetic Force Reduced to Absolute Measurement |url=http://21stcenturysciencetech.com/translations/gaussMagnetic.pdf |access-date=2009-10-21}}</ref> It described a new instrument that consisted of a permanent bar magnet suspended horizontally from a [[gold]] fibre. The difference in the oscillations when the bar was magnetised and when it was demagnetised allowed Gauss to calculate an absolute value for the strength of the Earth's magnetic field.<ref>{{cite web |url=http://www.ctsystems.eu/gauss.htm |archive-url=https://web.archive.org/web/20070930141937/http://www.ctsystems.eu/gauss.htm |archive-date=2007-09-30 |title=Magnetometer: The History |publisher=CT Systems |access-date=2009-10-21}}</ref>

The [[gauss (unit)|gauss]], the [[CGS]] unit of [[magnetic flux density]] was named in his honour, defined as one [[maxwell (unit)|maxwell]] per square centimeter; it equals 1×10<sup>−4</sup> [[tesla (unit)|tesla]] (the [[SI unit]]).<ref>{{cite web|title=Ferromagnetic Materials|url=http://phareselectronics.com/ferromagnetic-materials/|access-date=26 May 2015|archive-url=https://web.archive.org/web/20150627075722/http://phareselectronics.com/ferromagnetic-materials/|archive-date=27 June 2015|url-status=dead}}</ref>

[[Francis Ronalds]] and [[Charles Brooke (surgeon)|Charles Brooke]] independently invented magnetographs in 1846 that continuously recorded the magnet's movements using [[photography#Science and forensics|photography]], thus easing the load on observers.<ref>{{Cite journal|last=Ronalds|first=B.F.|date=2016|title=The Beginnings of Continuous Scientific Recording using Photography: Sir Francis Ronalds' Contribution|url=http://www.eshph.org/blog/2016/04/19/1642/|journal=European Society for the History of Photography|access-date=2 June 2016}}</ref> They were quickly utilised by [[Edward Sabine#The magnetic crusade|Edward Sabine]] and others in a global magnetic survey and updated machines were in use well into the 20th century.<ref>{{Cite book|title=Sir Francis Ronalds: Father of the Electric Telegraph|last=Ronalds|first=B.F.|publisher=Imperial College Press|year=2016|isbn=978-1-78326-917-4|location=London}}</ref><ref>{{Cite book|title=Encyclopedia of Geomagnetism and Paleomagnetism|editor1=David Gubbins|editor2=Emilio Herrero-Bervera|publisher=Springer|year=2007|isbn=978-1-4020-3992-8}}</ref>