Galvanometer
A galvanometer is an electromechanical
instrument used for detecting and indicating an
electric current. A galvanometer works as an
actuator, by producing a rotary deflection (of a
"pointer"), in response to electric current flowing
through a coil in a constant magnetic field. Early
galvanometers were not calibrated, but their later
developments were used as measuring
instruments, called ammeters, to measure the
current flowing through an electric circuit.
Galvanometers developed from the observation
that the needle of a magnetic compass is deflected
near a wire that has electric current flowing
through it, first described by Hans Christian An early D'Arsonval galvanometer showing magnet
Ørsted in 1820. They were the first instruments and rotating coil
used to detect and measure small amounts of
electric currents. André-Marie Ampère, who gave
mathematical expression to Ørsted's discovery and named the instrument after[1] 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.
Sensitive 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 also had widespread use as the visualising part in other kinds of analog meters, for
example in light meters, VU meters, etc., where they were used to measure and display the output of
other sensors. Today the main type of galvanometer mechanism, still in use, is the moving coil,
D'Arsonval/Weston type.
Contents
Operation
Uses
Modern uses
Past uses
History
Hans Oersted
Schweigger and Ampère
Poggendorff and Thomson
Georg Ohm
D'Arsonval and Deprez
Edward Weston
Taut-band movement
Types
Tangent galvanometer
Theory
Geomagnetic field measurement
Astatic galvanometer
Mirror galvanometer
Ballistic galvanometer
See also
References
External links
Operation
Modern galvanometers, of the D'Arsonval/Weston
type, are constructed with 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.
When a direct current (DC) flows through the
coil, the coil generates a magnetic field. This field
acts against the permanent magnet. The coil
twists, pushing against the spring, and moves the
pointer. The hand points at a scale indicating the
electric current. Careful design of the pole pieces
ensures that the magnetic field is uniform, so that Diagram of D'Arsonval/Weston type galvanometer. As
the angular deflection of the pointer is the current flows from + through the coil (the orange
proportional to the current. A useful meter part) to −, a magnetic field is generated in the coil.
generally contains provision for damping the This field is counteracted by the permanent magnet
and forces the coil to twist, moving the pointer, in
mechanical resonance of the moving coil and
relation to the field's strength caused by the flow of
pointer, so that the pointer settles quickly to its current.
position without oscillation.
The basic sensitivity of a meter might be, for instance, 100 microamperes full scale (with a voltage drop
of, say, 50 millivolts at full current). Such meters are often calibrated to read some other quantity that can
be converted to a current of that magnitude. The use of current dividers, often called shunts, allows a
meter to be calibrated to measure larger currents. A meter can be calibrated as a DC voltmeter if the
resistance of the coil is known by calculating the voltage required to generate a full scale current. A
meter can be configured to read other voltages by putting it in a voltage divider circuit. This is generally
done by placing a resistor in series with the meter coil. A meter can be used to read resistance by placing
it in series with a known voltage (a battery) and an adjustable resistor. In a preparatory step, the circuit is
completed and the resistor adjusted to produce full scale deflection. When an unknown resistor is placed
in series in the circuit the current will be less than full scale and an appropriately calibrated scale can
display the value of the previously unknown resistor.
These capabilities to translate different kinds of electric quantities, in to pointer movements, make the
galvanometer ideal for turning output of other sensors that outputs electricity (in some form or another),
into something that can be read by a human.
Because the pointer of the meter is usually a small distance above the scale of the meter, parallax error
can occur when the operator attempts to read the scale line that "lines up" with the pointer. To counter
this, some meters include a mirror along the markings of the principal scale. The accuracy of the reading
from a mirrored scale is improved by positioning one's head while reading the scale so that the pointer
and the reflection of the pointer are aligned; at this point, the operator's eye must be directly above the
pointer and any parallax error has been minimized.
Uses
Probably the largest use of galvanometers was of the
D'Arsonval/Weston type used in analog meters in electronic
equipment. Since the 1980s, galvanometer-type analog meter
movements have been displaced by analog to digital converters
(ADCs) for many uses. A digital panel meter (DPM) contains an
analog to digital converter and numeric display. The advantages
of a digital instrument are higher precision and accuracy, but
factors such as power consumption or cost may still favour
application of analog meter movements.
Modern uses
Most modern uses for the galvanometer mechanism are in
positioning and control systems. Galvanometer mechanisms are
divided into moving magnet and moving coil galvanometers; in
addition, they are divided into closed-loop and open-loop - or
resonant - types.
Closed-loop galvanometer-driven
laser scanning mirror
Mirror galvanometer systems are used as beam positioning or
beam steering elements in laser scanning systems. For example,
for material processing with high-power lasers, closed loop mirror galvanometer mechanisms are used
with servo control systems. These are typically high power galvanometers and the newest galvanometers
designed for beam steering applications can have frequency responses over 10 kHz with appropriate
servo technology. Closed-loop mirror galvanometers are also used in similar ways in stereolithography,
laser sintering, laser engraving, laser beam welding, laser TVs, laser displays and in imaging applications
such as retinal scanning with Optical Coherence Tomography (OCT). Almost all of these galvanometers
are of the moving magnet type. The closed loop is obtained measuring the position of the rotating axis
with an infrared emitter and 2 photodiodes. This feedback is an analog signal.
Open loop, or resonant mirror galvanometers, are mainly used in some types of laser-based bar-code
scanners, printing machines, imaging applications, military applications and space systems. Their non-
lubricated bearings are especially of interest in applications that require functioning in a high vacuum.
Moving coil type galvanometer mechanisms
(called 'voice coils' by hard disk manufacturers)
are used for controlling the head positioning
servos in hard disk drives and CD/DVD players,
in order to keep mass (and thus access times), as
low as possible.
Past uses
A major early use for galvanometers was for
finding faults in telecommunications cables. They
were superseded in this application late in the
20th century by time-domain reflectometers. A galvanometer mechanism (center part), used in an
automatic exposure unit of an 8 mm film camera,
Galvanometer mechanisms were also used to get together with a photoresistor (seen in the hole on top
readings from photoresistors in the metering of the leftpart).
mechanisms of film cameras (as seen in the
adjacent image).
In analog strip chart recorders such as used in electrocardiographs, electroencephalographs and
polygraphs, galvanometer mechanisms were used to position the pen. Strip chart recorders with
galvanometer driven pens may have a full scale frequency response of 100 Hz and several centimeters of
deflection.
History
Hans Oersted
The deflection of a magnetic compass needle by current in a wire was first described by Hans Oersted in
1820. The phenomenon was studied both for its own sake and as a means of measuring electric current.
Schweigger and Ampère
The earliest galvanometer was reported by Johann Schweigger at the University of Halle on 16
September 1820. André-Marie Ampère also contributed to its development. Early designs increased the
effect of the magnetic field generated by the current by using multiple turns of wire. The instruments
were at first called "multipliers" due to this common design feature.[2] The term "galvanometer," in
common use by 1836, was derived from the surname of Italian electricity researcher Luigi Galvani, who
in 1791 discovered that electric current would make a dead frog's leg jerk.
Poggendorff and Thomson
Originally, the instruments relied on the Earth's
magnetic field to provide the restoring force for
the compass needle. These were called "tangent"
galvanometers and had to be oriented before use.
Later instruments of the "astatic" type used
opposing magnets to become independent of the
Earth's field and would operate in any orientation.
The most sensitive form, the Thomson or mirror
galvanometer, was patented in 1858 by William
Thomson (Lord Kelvin) as an improvement of an
earlier design invented in 1826 by Johann
Christian Poggendorff. Thomson's design was
able to detect very rapid current changes by using
small magnets attached to a lightweight mirror, Thomson mirror galvanometer, patented in 1858.
suspended by a thread, instead of a compass
needle. The deflection of a light beam on the
mirror greatly magnified the deflection induced by small currents. Alternatively, the deflection of the
suspended magnets could be observed directly through a microscope.
Georg Ohm
The ability to measure quantitatively voltage and current allowed Georg Ohm, in 1827, to formulate
Ohm's Law – that the voltage across a conductor is directly proportional to the current through it.
D'Arsonval and Deprez
The early moving-magnet form of galvanometer had the disadvantage that it was affected by any
magnets or iron masses near it, and its deflection was not linearly proportional to the current. In 1882
Jacques-Arsène d'Arsonval and Marcel Deprez developed a form with a stationary permanent magnet and
a moving coil of wire, suspended by fine wires which provided both an electrical connection to the coil
and the restoring torque to return to the zero position. An iron tube between the magnet's pole pieces
defined a circular gap through which the coil rotated. This gap produced a consistent, radial magnetic
field across the coil, giving a linear response throughout the instrument's range. A mirror attached to the
coil deflected a beam of light to indicate the coil position. The concentrated magnetic field and delicate
suspension made these instruments sensitive; d'Arsonval's initial instrument could detect ten
microamperes.[3]
Edward Weston
Edward Weston extensively improved the design. He replaced the fine wire suspension with a pivot, and
provided restoring torque and electrical connections through spiral springs rather like those of a
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
light-weight 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.
D'Arsonval/Weston galvanometer (ca. 1900). Part of
Taut-band movement the magnet's left pole piece is broken out to show the
The taut-band movement is a modern coil.
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.[4]
Types
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 Weston galvanometer in portable
operating principle, the tangent law of magnetism, which states case
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 Jacob Nervander in 1834 (see J.J. Nervander, “Mémoire
sur un Galvanomètre à châssis cylindrique par lequel on obtient immédiatement et sans calcul la mesure
de l’intensité du courant électrique qui produit la déviation de l’aiguille aimantée,” Annales de Chimie et
de Physique (Paris), Tome 55, 156–184, 1834. and J. Venermo and A. Sihvola, "The tangent
galvanometer of Johan Jacob Nervander," IEEE Instrumentation & Measurement Magazine, vol. 11, no.
3, pp. 16-23, June 2008.) and in 1837 by Claude Pouillet.[5]
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.[6] 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.
Tangent Galvanometer
An 1850 Pouillet Tangent Top view of a
Tangent galvanometer tangent
Galvanometer on made by J. H. galvanometer made
display at Musée Bunnell Co. about 1950. The
d'histoire des around 1890. indicator needle of
sciences de la Ville the compass is
de Genève perpendicular to the
shorter, black
magnetic needle.
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:
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:
From tangent law, B = BH tan θ, i.e.
or
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.[7]
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,[8] 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.[9] 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.
Nobili's astatic galvanometer
Galvanometer on Detail of an
display at Musée astatic
d'histoire des galvanometer.
sciences de la Ville
de Genève
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. In reality it is an integrator, unlike a current-measuring galvanometer, the moving
part has a large moment of inertia that gives it a long oscillation period. 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
References
1. Schiffer, Michael Brian. (2008)"Electromagnetism Revealed," Power Struggles: Scientific
Authority and the Creation of Practical Electricity Before Edison. Page 24.
2. "Schweigger Multiplier – 1820" (https://nationalmaglab.org/education/magnet-academy/histo
ry-of-electricity-magnetism/museum/schweigger-multiplier). Maglab. National High Magnetic
Field Laboratory. Retrieved 17 October 2017.
3. Keithley, Joseph F. (1999). The story of electrical and magnetic measurements: from 500
B.C. to the 1940s. John Wiley and Sons. pp. 196–198. ISBN 0-7803-1193-0.
4. http://www.dictionarycentral.com/definition/taut-band-meter.html
5. Pouillet (1837). "Mémoire sur la pile de Volta et sur la loi générale de l'intensité que
prennent les courrants, soit qu'ils proviennent d'un seul élément, soit qu'ils proviennent
d'une pile à grande ou à petite tension" (https://www.biodiversitylibrary.org/item/112006#pag
e/273/mode/1up) [Memoir on the Voltaic pile [i.e., battery] and on the general law of the
intensity that currents assume, whether they come from a single element or they come from
a pile of high or low voltage]. Comptes rendus (in French). 4: 267–279.
6. Greenslade, Jr., Thomas B. "Tangent Galvanometer" (http://physics.kenyon.edu/EarlyAppar
atus/Electrical_Measurements/Tangent_Galvanometer/Tangent_Galvanometer.html).
Kenyon College. Retrieved 26 April 2016.
7. "Theory" (http://prugalvanometer.weebly.com/theory.html). GALVANOMETER. Retrieved
5 April 2017.
8. Nobili, Leopoldo (1825). "Sur un nouveau galvanomètre présenté à l'Académie des
Sciences" (https://babel.hathitrust.org/cgi/pt?id=pst.000052859663;view=1up;seq=125) [On
a new galvanometer presented at the Academy of Sciences]. Bibliothèque universelle (in
French). 29: 119–125.
9. Greenslade, Thomas B., Jr. "Instruments for Natural Philosophy — Astatic Galvanometer"
(http://physics.kenyon.edu/EarlyApparatus/Electrical_Measurements/Astatic_Galvanometer/
Astatic_Galvanometer.html). Kenyon College. Retrieved 6 November 2019.
External links
Galvanometer - Interactive Java Tutorial (https://nationalmaglab.org/education/magnet-acad
emy/watch-play/interactive/galvanometer) National High Magnetic Field Laboratory
Selection of historic galvanometer (http://vlp.mpiwg-berlin.mpg.de/technology/search?-max
=10&-title=1&-op_varioid=numerical&varioid=7) in the Virtual Laboratory of the Max Planck
Institute for the History of Science
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