A mirror galvanometer is an

ammeter An ammeter (abbreviation of ''ampere meter'') is an measuring instrument, instrument used to measure the electric current, current in a Electrical circuit, circuit. Electric currents are measured in amperes (A), hence the name. For direct measure ...

that indicates it has sensed an electric

current Currents, Current or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (hydr ...

by deflecting a light beam with a

mirror A mirror, also known as a looking glass, is an object that Reflection (physics), reflects an image. Light that bounces off a mirror forms an image of whatever is in front of it, which is then focused through the lens of the eye or a camera ...

. The beam of light projected on a scale acts as a long massless pointer. In 1826,

Johann Christian Poggendorff Johann Christian Poggendorff (29 December 1796 – 24 January 1877) was a German physicist born in Hamburg. By far the greater and more important part of his work related to electricity and magnetism. Poggendorff is known for his electrostatic mo ...

developed the mirror galvanometer for detecting electric currents. The apparatus is also known as a ''spot galvanometer'' after the spot of light produced in some models. Mirror galvanometers were used extensively in scientific instruments before reliable, stable

electronic amplifier An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a Signal (information theory), signal (a time-varying voltage or Electric current, current). It is a two-port network, two-port ...

s were available. The most common uses were as recording equipment for

seismometer A seismometer is an instrument that responds to ground displacement and shaking such as caused by quakes, volcanic eruptions, and explosions. They are usually combined with a timing device and a recording device to form a seismograph. The out ...

s and

submarine cables Submarine cable is any electrical cable that is laid on the seabed, although the term is often extended to encompass cables laid on the bottom of large freshwater bodies of water. Examples include: *Submarine communications cable *Submarine power ...

used for telegraphy. In modern times, the term ''mirror galvanometer'' is also used for devices that move

laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word ''laser'' originated as an acronym for light amplification by stimulated emission of radi ...

beams by rotating a mirror through a galvanometer set-up, often with a servo-like control loop. The name is often abbreviated as ''galvo''.

Kelvin's galvanometer

The mirror galvanometer was improved significantly by William Thomson, later to become Lord Kelvin. He coined the term ''mirror galvanometer'' and patented the device in 1858. Thomson intended the instrument to read weak signal

s on very long

submarine telegraph cable A submarine communications cable is a cable laid on the seabed between land-based stations to carry telecommunication signals across stretches of ocean and sea. The first submarine communications cables were laid beginning in the 1850s and car ...

s. This instrument was far more sensitive than any which preceded it, enabling the detection of the slightest defect in the core of a cable during its manufacture and submersion. Thomson decided that he needed an extremely sensitive instrument after he took part in the failed attempt to lay a

transatlantic telegraph cable Transatlantic telegraph cables were undersea cables running under the Atlantic Ocean for telegraph communications. Telegraphy is a largely obsolete form of communication, and the cables have long since been decommissioned, but telephone and dat ...

in 1857. He worked on the device while waiting for a new expedition the following year. He first looked at improving a galvanometer used by

Hermann von Helmholtz Hermann Ludwig Ferdinand von Helmholtz (; ; 31 August 1821 – 8 September 1894; "von" since 1883) was a German physicist and physician who made significant contributions in several scientific fields, particularly hydrodynamic stability. The ...

to measure the speed of nerve signals in 1849. Helmholtz's galvanometer had a mirror fixed to the moving needle, which was used to project a beam of light onto the opposite wall, thus greatly amplifying the signal. Thomson intended to make this more sensitive by reducing the mass of the moving parts, but in a flash of inspiration while watching the light reflected from his

monocle A monocle is a type of corrective lens used to correct or enhance the visual perception in only one eye. It consists of a circular lens placed in front of the eye and held in place by the eye socket itself. Often, to avoid losing the monoc ...

suspended around his neck, he realised that he could dispense with the needle and its mounting altogether. He instead used a small piece of mirrored glass with a small piece of magnetised steel glued on the back. This was suspended by a thread in the magnetic field of the fixed sensing coil. In a hurry to try the idea, Thomson first used a hair from his dog, but later used a silk thread from the dress of his niece Agnes.Lindley, David, ''Degrees Kelvin: A Tale of Genius, Invention, and Tragedy'', pp. 132–133, Joseph Henry Press, 2004 . The following is adapted from a contemporary account of Thomson's instrument:

Moving coil galvanometer

Moving coil galvanometer was developed independently by Marcel Deprez and Jacques-Arsène d'Arsonval about 1880. Deprez's galvanometer was developed for high currents, while D'Arsonval designed his to measure weak currents. Unlike in the Kelvin's galvanometer, in this type of galvanometer the magnet is stationary and the coil is suspended in the magnet gap. The mirror attached to the coil frame rotates together with it. This form of instrument can be more sensitive and accurate and it replaced the Kelvin's galvanometer in most applications. The moving coil galvanometer is practically immune to ambient magnetic fields. Another important feature is self-damping generated by the electro-magnetic forces due to the currents induced in the coil by its movements the magnetic field. These are proportional to the angular velocity of the coil.

Modern uses

In modern times, high-speed mirror

galvanometer A galvanometer is an electromechanical measuring instrument for electric current. Early galvanometers were uncalibrated, but improved versions, called ammeters, were calibrated and could measure the flow of current more precisely. Galvanomet ...

s are employed in laser light shows to move the laser beams and produce colorful geometric patterns in fog around the audience. Such high speed mirror galvanometers have proved to be indispensable in industry for laser marking systems for everything from laser etching hand tools, containers, and parts to batch-coding semiconductor wafers in

semiconductor device fabrication Semiconductor device fabrication is the process used to manufacture semiconductor devices, typically integrated circuits (ICs) such as microprocessors, microcontrollers, and memories (such as Random-access memory, RAM and flash memory). It is a ...

. They typically control X and Y directions on Nd:YAG and CO₂ laser markers to control the position of the infrared power laser spot.

Laser ablation Laser ablation or photoablation (also called laser blasting) is the process of removing material from a solid (or occasionally liquid) surface by irradiating it with a laser beam. At low laser flux, the material is heated by the absorbed laser ...

laser beam machining Laser beam machining (LBM) is a form of machining that uses heat directed from a laser beam. This process uses thermal energy to remove material from metallic or nonmetallic surfaces. The high frequency of monochromatic light will fall on the sur ...

and wafer dicing are all industrial areas where high-speed mirror galvanometers can be found. This moving coil galvanometer is mainly used to measure very feeble or low currents of order 10⁻⁹ A. To linearise the magnetic field across the coil throughout the galvanometer's range of movement, the d'Arsonval design of a soft iron cylinder is placed inside the coil without touching it. This gives a consistent radial field, rather than a parallel linear field.

References

External links

{{Commons category, Mirror galvanometers
Mirror Galvanometer - Interactive Java Tutorial
National High Magnetic Field Laboratory Galvanometers Optical instruments

Kelvin's galvanometer

Moving coil galvanometer

Modern uses

See also

References

Further reading

External links