Editing Galvanometer (section)

==Types==
There are broadly two types of galvanometers. Some galvanometers use a solid pointer on a scale to show measurements; other very sensitive types use a miniature mirror and a beam of light to provide mechanical amplification of low-level signals.

===Tangent galvanometer===
A tangent galvanometer is an early [[measuring instrument]] used for the measurement of [[electric current]]. It works by using a [[compass]] needle to compare a [[magnetic field]] generated by the unknown current to the magnetic field of the Earth. It gets its name from its operating principle, the tangent law of magnetism, which states that the [[tangent]] of the angle a compass needle makes is proportional to the ratio of the strengths of the two perpendicular magnetic fields. It was first described by [[Johan Jakob Nervander]] in 1834.<ref>{{cite journal |last1=Nervander |first1=J.J. |title=Mémoire sur un Galvanomètre à châssis cylindrique par lequel on obtient immédiatement et sans calcul la mesure de l'intensité du courant électrique qui produit la déviation de l'aiguille aimantée |journal=Annales de Chimie et de Physique (Paris) |date=1834 |volume=55 |pages=156–184 |url=https://archive.org/details/s3id13208240/page/156/mode/2up |trans-title=Memoir on a cylindrical-frame galvanometer by which one obtains immediately and without calculation the measurement of the intensity of the electric current which produces the deflection of the magnetic needle |language=French}}</ref><ref>{{cite journal |last1=Pouillet |first1=Claude |title=Mémoire sur la pile de Volta et sur la loi générale de l'intensité que prennent les courrants, soit qu'ils proviennent d'un seul élément, soit qu'ils proviennent d'une pile à grande ou à petite tension |journal=Comptes rendus hebdomadaires des séances de l'Académie des sciences |date=1837 |volume=4 |pages=267–279 |url=https://www.biodiversitylibrary.org/item/112006#page/273/mode/1up |trans-title=Memoir on the Voltaic pile [i.e., battery] and on the general law of the intensity that currents assume, whether they come from a single element or they come from a pile of high or low voltage |language=French}}</ref><ref>Moritz Jacobi calibrated galvanometers by measuring the amount of water decomposed by electric currents:  {{cite journal |last1=Jacobi |first1=M. |title=Ueber das chemische und das magnetische-Galvanometer |journal=Annalen der Physik und Chemie |date=1839 |volume=48 |pages=26–57 |url=https://babel.hathitrust.org/cgi/pt?id=wu.89048351787&view=1up&seq=684&skin=2021 |series=2nd series |issue=9 |doi=10.1002/andp.18391240903 |bibcode=1839AnP...124...26J |trans-title=On the chemical and magnetic galvanometer |language=German}}</ref><ref>{{cite journal |last1=Venermo |first1=J. |last2=Sihvola |first2=A. |title=The tangent galvanometer of Johan Jacob Nervander |journal=IEEE Instrumentation & Measurement Magazine |date=June 2008 |volume=11 |issue=3 |pages=16–23|doi=10.1109/MIM.2008.4534374 |s2cid=27081490 }}</ref>

A tangent galvanometer consists of a coil of insulated copper wire wound on a circular non-magnetic frame. The frame is mounted vertically on a horizontal base provided with levelling screws. The coil can be rotated on a vertical axis passing through its centre. A compass box is mounted horizontally at the centre of a circular scale. It consists of a tiny, powerful magnetic needle pivoted at the centre of the coil. The magnetic needle is free to rotate in the horizontal plane. The circular scale is divided into four quadrants. Each quadrant is graduated from 0° to 90°. A long thin aluminium pointer is attached to the needle at its centre and at right angle to it. To avoid errors due to parallax, a plane mirror is mounted below the compass needle.

In operation, the instrument is first rotated until the magnetic field of the Earth, indicated by the compass needle, is parallel with the plane of the coil. Then the unknown current is applied to the coil. This creates a second magnetic field on the axis of the coil, perpendicular to the Earth's magnetic field. The compass needle responds to the [[vector sum]] of the two fields and deflects to an angle equal to the tangent of the ratio of the two fields. From the angle read from the compass's scale, the current could be found from a table.<ref>{{cite web |title=Tangent Galvanometer |last=Greenslade Jr. |first=Thomas B. |publisher=Kenyon College |url=http://physics.kenyon.edu/EarlyApparatus/Electrical_Measurements/Tangent_Galvanometer/Tangent_Galvanometer.html |access-date=26 April 2016 |archive-date=4 June 2011 |archive-url=https://web.archive.org/web/20110604155352/http://physics.kenyon.edu/EarlyApparatus/Electrical_Measurements/Tangent_Galvanometer/Tangent_Galvanometer.html |url-status=dead }}</ref> The current supply wires have to be wound in a small helix, like a pig's tail, otherwise the field due to the wire will affect the compass needle and an incorrect reading will be obtained.
<gallery caption="Tangent Galvanometer" mode="packed" heights="150" style="font-size:88%; line-height:130%;">
File:Sine and Tangent Galvanometer-MHS 98-IMG 3874-gradient.jpg|An 1850 [[Claude Pouillet|Pouillet]] Tangent Galvanometer on display at [[Musée d'histoire des sciences de la Ville de Genève]]
File:Western Union standard galvanometer.jpg|alt=Drawing. The prominent feature is a vertical ring seen from the front. It is mounted on a horizontal disk that has electrical connectors. A horizontal compass is mounted at the center of the ring.|Tangent galvanometer made by J. H. Bunnell Co. around 1890.
File:Tangent galvanometer Philip-Harris top1.jpg|alt=Photograph. The most prominent feature is a horizontal circular compass case that is seen from above. The compass is centered inside of a black ring with a square cross-section. The compass and ring are supported on a brass tripod that has leveling screws as its feet.|Top view of a tangent galvanometer made about 1950. The indicator needle of the compass is perpendicular to the shorter, black magnetic needle.
</gallery>

====Theory====
The galvanometer is oriented so that the plane of the coil is vertical and aligned along parallel to the horizontal component {{math|''B<sub>H</sub>''}} of the Earth's magnetic field (i.e. parallel to the local "magnetic meridian"). When an electric current flows through the galvanometer coil, a second magnetic field {{math|''B''}} is created. At the center of the coil, where the compass needle is located, the coil's field is perpendicular to the plane of the coil. The magnitude of the coil's field is:
:<math>B={\mu_0 nI\over 2r}\,</math>
where {{math|''I''}} is the current in [[ampere (unit)|ampere]]s, {{math|''n''}} is the number of turns of the coil and {{math|''r''}} is the radius of the coil. These two perpendicular magnetic fields add [[Parallelogram of force|vectorially]], and the compass needle points along the direction of their resultant {{math|''B<sub>H</sub>+B''}}. The current in the coil causes the compass needle to rotate by an angle {{math|''θ''}}:
:<math>\theta = \tan^{-1} \frac{B}{B_H}\,</math>
From tangent law, {{math|''B {{=}} B<sub>H </sub>''tan'' θ''}}, i.e.
:<math>{\mu_0 nI\over 2r} = B_H \tan\theta\,</math>
or
:<math>I=\left(\frac{2rB_H}{\mu_0 n}\right)\tan\theta\,</math>
or {{math|''I {{=}} K ''tan'' θ''}}, where {{math|''K''}} is called the Reduction Factor of the tangent galvanometer.

One problem with the tangent galvanometer is that its resolution degrades at both high currents and low currents.  The maximum resolution is obtained when the value of {{math|''θ''}} is 45°. When the value of {{math|''θ''}} is close to 0° or 90°, a large percentage change in the current will only move the needle a few degrees.<ref>{{Cite web|url=http://prugalvanometer.weebly.com/theory.html|title=Theory|website=GALVANOMETER|access-date=2017-04-05}}</ref>

====Geomagnetic field measurement====
A tangent galvanometer can also be used to measure the magnitude of the horizontal component of the [[geomagnetic field]]. When used in this way, a low-voltage power source, such as a battery, is connected in series with a [[rheostat]], the galvanometer, and an [[ammeter]]. The galvanometer is first aligned so that the coil is parallel to the geomagnetic field, whose direction is indicated by the compass when there is no current through the coils. The battery is then connected and the rheostat is adjusted until the compass needle deflects 45 degrees from the geomagnetic field, indicating that the magnitude of the magnetic field at the center of the coil is the same as that of the horizontal component of the geomagnetic field. This field strength can be calculated from the current as measured by the ammeter, the number of turns of the coil, and the radius of the coils.

===Astatic galvanometer===
Unlike the tangent galvanometer, the ''[[Astatic needles|astatic]] galvanometer'' does not use the Earth's magnetic field for measurement, so it does not need to be oriented with respect to the Earth's field, making it easier to use. Developed by [[Leopoldo Nobili]] in 1825,<ref>{{cite journal|last1=Nobili|first1=Leopoldo|title=Sur un nouveau galvanomètre présenté à l'Académie des Sciences|journal=Bibliothèque universelle|date=1825|volume=29|pages=119–125|url=https://babel.hathitrust.org/cgi/pt?id=pst.000052859663;view=1up;seq=125|trans-title=On a new galvanometer presented at the Academy of Sciences|language=fr}}</ref> it consists of two magnetized needles parallel to each other but with the magnetic poles reversed. These needles are suspended by a single silk thread.<ref>{{cite web |url=http://physics.kenyon.edu/EarlyApparatus/Electrical_Measurements/Astatic_Galvanometer/Astatic_Galvanometer.html |title=Instruments for Natural Philosophy — Astatic Galvanometer |publisher=Kenyon College |author=Greenslade, Thomas B. Jr. |access-date=6 November 2019 |archive-date=7 March 2018 |archive-url=https://web.archive.org/web/20180307212444/http://physics.kenyon.edu/EarlyApparatus/Electrical_Measurements/Astatic_Galvanometer/Astatic_Galvanometer.html |url-status=dead }}</ref> The lower needle is inside a vertical current sensing coil of wire and is deflected by the magnetic field created by the passing current, as in the tangent galvanometer above.  The purpose of the second needle is to cancel the dipole moment of the first needle, so the suspended armature has no net [[magnetic dipole moment]], and thus is not affected by the earth's magnetic field.  The needle's rotation is opposed by the torsional elasticity of the suspension thread, which is proportional to the angle.

<gallery caption="Nobili's astatic galvanometer" mode="packed" heights="150" style="font-size:88%; line-height:130%;">
File:Galvanometer-MHS 229-IMG 3875-gradient.jpg|Galvanometer on display at [[Musée d'histoire des sciences de la Ville de Genève]]
File:Astatic Galvanometer brass and ivory.jpg|Detail of an astatic galvanometer.
</gallery>

===Mirror galvanometer===
{{main article|Mirror galvanometer}}
To achieve higher sensitivity to detect extremely small currents, the [[mirror galvanometer]] substitutes a lightweight mirror for the pointer.  It consists of horizontal magnets suspended from a fine fiber, inside a vertical coil of wire, with a mirror attached to the magnets.  A beam of light reflected from the mirror falls on a graduated scale across the room, acting as a long mass-less pointer. The mirror galvanometer was used as the receiver in the first trans-Atlantic [[submarine communications cable|submarine telegraph cables]] in the 1850s, to detect the extremely faint pulses of current after their thousand-mile journey under the Atlantic. In a device called an [[oscillograph]], the moving beam of light is used, to produce graphs of current versus time, by recording measurements on photographic film. The [[string galvanometer]] is a type of mirror galvanometer so sensitive that it was used to make the first [[electrocardiogram]] of the electrical activity of the human heart.

===Ballistic galvanometer===
{{main article|Ballistic galvanometer}}
A ballistic galvanometer is a type of sensitive galvanometer for measuring the quantity of [[Electric charge|charge]] discharged through it. It is an [[integrator]], by virtue of the long [[time constant]] of its response, unlike a current-measuring galvanometer. The moving part has a large [[moment of inertia]] that gives it an [[oscillation]] period long enough to make the integrated measurement. It can be either of the moving coil or moving magnet type; commonly it is a mirror galvanometer.