Editing Galvanometer (section)

====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>