A mirror galvanometer

Galvanometer

A galvanometer is a type of ammeter: an instrument for detecting and measuring electric current. It is an analog electromechanical transducer that produces a rotary deflection of some type of pointer in response to electric current flowing through its coil in a magnetic field. .Galvanometers were...

is a mechanical meter that senses electric current, except that instead of moving a needle, it moves a mirror

Mirror

A mirror is an object that reflects light or sound in a way that preserves much of its original quality prior to its contact with the mirror. Some mirrors also filter out some wavelengths, while preserving other wavelengths in the reflection...

. The mirror reflects a beam of light, which projects onto a meter, and acts as a long, weightless, massless pointer. In 1826, Johann Christian Poggendorff

Johann Christian Poggendorff

Johann Christian Poggendorff , 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 motor which is analogous to Wilhelm Holtz's electrostatic machine...

 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

Electronic amplifier

An electronic amplifier is a device for increasing the power of a signal.It does this by taking energy from a power supply and controlling the output to match the input signal shape but with a larger amplitude...

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

Seismometer

Seismometers are instruments that measure motions of the ground, including those of seismic waves generated by earthquakes, volcanic eruptions, and other seismic sources...

s and submarine cable

Submarine cable

Submarine cable may refer to:*Submarine communications cable*Submarine power cable...

s used for telegraphy.

In modern times, the term mirror galvanometer is also used for devices that move laser beams by rotating a mirror through a galvanometer set-up.

Kelvin's galvanometer

The mirror galvanometer was later improved by William Thomson

William Thomson, 1st Baron Kelvin

William Thomson, 1st Baron Kelvin OM, GCVO, PC, PRS, PRSE, was a mathematical physicist and engineer. At the University of Glasgow he did important work in the mathematical analysis of electricity and formulation of the first and second laws of thermodynamics, and did much to unify the emerging...

, later to become Lord Kelvin. He would patent the device in 1858.

Thomson reacted to the need for an instrument that could indicate with sensibility all the variations of the current in a long cable. This instrument was far more sensitive

Sensitivity (electronics)

The sensitivity of an electronic device, such as a communications system receiver, or detection device, such as a PIN diode, is the minimum magnitude of input signal required to produce a specified output signal having a specified signal-to-noise ratio, or other specified criteria.Sensitivity is...

 than any which preceded it, enabling the detection of the slightest defect in the core of a cable during its manufacture and submersion. Moreover, it proved the best apparatus for receiving messages through a long cable.

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

Jacques-Arsène d'Arsonval

Jacques-Arsène d'Arsonval was born in La Porcherie and was a French physician, physicist and inventor of the moving-coil galvanometer and probably of the thermocouple ammeter...

 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 mirror galvanometers

In modern times, high-speed mirror galvanometer

Galvanometer

A galvanometer is a type of ammeter: an instrument for detecting and measuring electric current. It is an analog electromechanical transducer that produces a rotary deflection of some type of pointer in response to electric current flowing through its coil in a magnetic field. .Galvanometers were...

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

Wafer (electronics)

A wafer is a thin slice of semiconductor material, such as a silicon crystal, used in the fabrication of integrated circuits and other microdevices...

 in semiconductor device fabrication. They typically control X and Y directions

Cartesian coordinate system

A Cartesian coordinate system specifies each point uniquely in a plane by a pair of numerical coordinates, which are the signed distances from the point to two fixed perpendicular directed lines, measured in the same unit of length...

 on Nd:YAG and CO2 laser markers to control the position of the infrared power laser spot. Laser ablation

Laser ablation

Laser ablation is the process of removing material from a solid surface by irradiating it with a laser beam. At low laser flux, the material is heated by the absorbed laser energy and evaporates or sublimates. At high laser flux, the material is typically converted to a plasma...

, laser beam machining

Laser beam machining

Laser Beam machining is an unconventional machining process in which a beam of highly coherent light called a Laser is directed towards the work piece for machining. Since the rays of a laser beam are monochromatic and parallel it can be focused to a very small diameter and can produce energy as...

 and wafer dicing

Wafer dicing

Wafer dicing is the process by which die are separated from a wafer of semiconductor following the processing of the wafer. The dicing process can be accomplished by scribing and breaking, by mechanical sawing or by laser cutting...

 are all industrial areas where high-speed mirror galvanometers can be found. Closer to home, mirror galvanometers are located in most retail outlets, warehouses, and parcel delivery service providers, in the form of barcode reader

Barcode reader

A barcode reader is an electronic device for reading printed barcodes. Like a flatbed scanner, it consists of a light source, a lens and a light sensor translating optical impulses into electrical ones...

s for Universal Product Code

Universal Product Code

The Universal Product Code is a barcode symbology , that is widely used in North America, and in countries including the UK, Australia, and New Zealand for tracking trade items in stores. Its most common form, the UPC-A, consists of 12 numerical digits, which are uniquely assigned to each trade item...

s and other forms of barcode

Barcode

A barcode is an optical machine-readable representation of data, which shows data about the object to which it attaches. Originally barcodes represented data by varying the widths and spacings of parallel lines, and may be referred to as linear or 1 dimensional . Later they evolved into rectangles,...

s.

External links

• Mirror Galvanometer - Interactive Java Tutorial National High Magnetic Field Laboratory