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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Galvanometer | 3/4 | https://en.wikipedia.org/wiki/Galvanometer | reference | science, encyclopedia | 2026-05-05T09:42:36.412179+00:00 | kb-cron |
Edward Weston extensively improved the design of the galvanometer. He substituted the fine wire suspension with a pivot and provided restoring torque and electrical connections through spiral springs rather than through the traditional wristwatch balance wheel hairspring. He developed a method of stabilizing the magnetic field of the permanent magnet, so the instrument would have consistent accuracy over time. He replaced the light beam and mirror with a knife-edge pointer that could be read directly. A mirror under the pointer, in the same plane as the scale, eliminated parallax observation error. To maintain the field strength, Weston's design used a very narrow circumferential slot through which the coil moved, with a minimal air-gap. This improved linearity of pointer deflection with respect to coil current. Finally, the coil was wound on a lightweight form made of conductive metal, which acted as a damper. By 1888, Edward Weston had patented and brought out a commercial form of this instrument, which became a standard electrical equipment component. It was known as a "portable" instrument because it was affected very little by mounting position or by transporting it from place to place. This design is almost universally used in moving-coil meters today. Initially, laboratory instruments relying on the Earth's own magnetic field to provide restoring force for the pointer, galvanometers were developed into compact, rugged, sensitive portable instruments essential to the development of electro-technology.
=== Taut-band movement === The taut-band movement is a modern development of the D'Arsonval-Weston movement. The jewel pivots and hairsprings are replaced by tiny strips of metal under tension. Such a meter is more rugged for field use.
== 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. 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. 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.
==== Theory ==== The galvanometer is oriented so that the plane of the coil is vertical and aligned along parallel to the horizontal component BH 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 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:
B
=
μ
0
n
I
2
r
{\displaystyle B={\mu _{0}nI \over 2r}\,}
where I is the current in amperes, n is the number of turns of the coil and r is the radius of the coil. These two perpendicular magnetic fields add vectorially, and the compass needle points along the direction of their resultant BH+B. The current in the coil causes the compass needle to rotate by an angle θ:
θ
=
tan
−
1
B
B
H
{\displaystyle \theta =\tan ^{-1}{\frac {B}{B_{H}}}\,}
From tangent law, B = BH tan θ, i.e.
μ
0
n
I
2
r
=
B
H
tan
θ
{\displaystyle {\mu _{0}nI \over 2r}=B_{H}\tan \theta \,}
or
I
=
(
2
r
B
H
μ
0
n
)
tan
θ
{\displaystyle I=\left({\frac {2rB_{H}}{\mu _{0}n}}\right)\tan \theta \,}
or I = K tan θ, where 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 θ is 45°. When the value of θ is close to 0° or 90°, a large percentage change in the current will only move the needle a few degrees.