Operation and Construction of Galvanometers
– Galvanometers are electromechanical measuring instruments for electric current.
– They work by deflecting a pointer in response to an electric current flowing through a coil in a constant magnetic field.
– Galvanometers can be thought of as a kind of actuator.
– They were the first instruments used to detect and measure small amounts of current.
– Galvanometers have been essential for the development of science and technology in many fields.
– Modern galvanometers are of the DArsonval/Weston type.
– They consist of a small pivoting coil of wire, called a spindle, in the field of a permanent magnet.
– The coil is attached to a thin pointer that traverses a calibrated scale.
– A tiny torsion spring pulls the coil and pointer to the zero position.
– Careful design of the pole pieces ensures that the magnetic field is uniform.
Calibration and Uses of Galvanometers
– Galvanometers are often calibrated to read some other quantity that can be converted to a current of a specific magnitude.
– Current dividers, known as shunts, allow a meter to be calibrated to measure larger currents.
– A galvanometer can be calibrated as a DC voltmeter if the resistance of the coil is known.
– It can be configured to read other voltages by placing a resistor in series with the coil.
– A galvanometer can be used to read resistance by placing it in series with a known voltage and an adjustable resistor.
– Galvanometers were widely used in analog meters in electronic equipment until the 1980s.
– They have been replaced by analog-to-digital converters (ADCs) in many applications.
– Modern uses of galvanometers are mainly in positioning and control systems.
– They are used in laser scanning systems, material processing with high-power lasers, and imaging applications.
– Galvanometers are also used in hard disk drives and CD/DVD players for head positioning.
Modern Applications of Galvanometers
– Galvanometer mechanisms are used in laser scanning systems for beam positioning and steering.
– They are used in material processing, stereolithography, laser engraving, and imaging applications.
– Closed-loop mirror galvanometers are used in retinal scanning and laser displays.
– Open-loop or resonant mirror galvanometers are used in bar-code scanners, printing machines, and military applications.
– Moving coil galvanometers are used for head positioning in hard disk drives and CD/DVD players.
History and Early Designs of Galvanometers
– Hans Christian Ørsted described the deflection of a magnetic compass needle by current in a wire in 1820.
– Johann Schweigger and André-Marie Ampère contributed to the development of the galvanometer.
– Luigi Galvani’s discovery of electric current making a dead frog’s leg jerk inspired the term ‘galvanometer.’
– Thomson mirror galvanometer, patented in 1858, was able to detect rapid current changes.
– Georg Ohm formulated Ohm’s law in 1827, thanks to the ability to measure voltage and current.
– Early galvanometers used multiple turns of wire to increase the magnetic field effect.
– Tangent galvanometers relied on Earth’s magnetic field and had to be oriented before use.
– Astatic galvanometers used opposing magnets to become independent of Earth’s field.
– Johann Christian Poggendorff invented an early mirror galvanometer in 1826.
– Hermann von Helmholtz invented the astatic galvanometer in 1849.
– Edward Weston replaced the fine wire suspension with a pivot and spiral springs for restoring torque and electrical connections.
– He stabilized the magnetic field of the permanent magnet for consistent accuracy.
– Weston’s design used a knife-edge pointer and a mirror to eliminate parallax observation error.
– A narrow circumferential slot improved linearity of pointer deflection with respect to coil current.
– The coil was wound on a lightweight conductive metal form, acting as a damper.
– The taut-band movement is a modern development of the DArsonval-Weston movement.
– It replaces jewel pivots and hairsprings with tiny strips of metal under tension.
– This type of galvanometer is more rugged for field use.
Types of Galvanometers
– Galvanometers can use a solid pointer on a scale or a miniature mirror and a beam of light for mechanical amplification.
– Tangent galvanometer is an early measuring instrument that compares magnetic fields to measure electric current.
– It works based on the tangent law of magnetism and was first described by Johan Jakob Nervander in 1834.
– The instrument consists of a coil of insulated copper wire, a compass needle, and a circular scale.
– The current is determined by measuring the angle of deflection of the compass needle.
– Astatic Galvanometer
– Developed by Leopoldo Nobili in 1825
– Consists of two magnetized needles with reversed magnetic poles
– Needles are suspended by a silk thread
– The lower needle is deflected by the passing current, while the second needle cancels out the magnetic dipole moment
– The rotation of the needles is opposed by the torsional elasticity of the suspension thread
– Mirror Galvanometer
– Substitutes a lightweight mirror for the pointer to achieve higher sensitivity
– Consists of horizontal magnets suspended from a fine fiber, with a mirror attached
– A beam of light reflected from the mirror falls on a graduated scale, acting as a pointer
– Used in the first trans-Atlantic submarine telegraph cables
– Can be used in oscillographs to produce graphs of current versus time
– Ballistic Galvanometer
– Used to measure the quantity of charge discharged through it
– An integrator with a long time constant of response
– Can be of the moving coil or moving magnet type
– Often a type of mirror galvanometer
– Used Source: https://en.wikipedia.org/wiki/D%27Arsonval_galvanometer
This article needs additional citations for verification. (March 2009) |
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.
A galvanometer works by deflecting a pointer in response to an electric current flowing through a coil in a constant magnetic field. Galvanometers can be thought of as a kind of actuator.
Galvanometers came from the observation, first noted by Hans Christian Ørsted in 1820, that a magnetic compass's needle deflects when near a wire having electric current. They were the first instruments used to detect and measure small amounts of current. André-Marie Ampère, who gave mathematical expression to Ørsted's discovery, named the instrument after the Italian electricity researcher Luigi Galvani, who in 1791 discovered the principle of the frog galvanoscope – that electric current would make the legs of a dead frog jerk.
Galvanometers have been essential for the development of science and technology in many fields. For example, in the 1800s they enabled long-range communication through submarine cables, such as the earliest transatlantic telegraph cables, and were essential to discovering the electrical activity of the heart and brain, by their fine measurements of current.
Galvanometers have also been used as the display components of other kinds of analog meters (e.g., light meters and VU meters), capturing the outputs of these meters' sensors. Today, the main type of galvanometer still in use is the D'Arsonval/Weston type.
