Editing Galvanometer (section)

===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>