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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Galvanometer | 4/4 | https://en.wikipedia.org/wiki/Galvanometer | reference | science, encyclopedia | 2026-05-05T09:42:36.412179+00:00 | kb-cron |
==== 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 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, 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. 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.
=== 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 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 ===
A ballistic galvanometer is a type of sensitive galvanometer for measuring the quantity of 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.
== See also == Vibration galvanometer Thermo galvanometer String galvanometer History of electrochemistry
== References ==
== External links ==
Galvanometer - Interactive Java Tutorial National High Magnetic Field Laboratory Selection of historic galvanometer in the Virtual Laboratory of the Max Planck Institute for the History of Science The History Corner: The Galvanometer by Nick Joyce and David Baker, April 1, 2008, Ass. of Physological Science. Retrieved February 26, 2022. Moving Coil Galvanometer