Galvanometer and earths magnetic field by shubham wasule 7566220325
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Galvanometer
D'Arsonval/Weston galvanometer movement - with the moving coil shown in orange.
A galvanometeris a type of sensitiveammeter:an instrument for detectingelectric current.It is
ananalogelectromechanicalactuatorthat produces a rotary deflection of some type of pointer in response
toelectric currentflowing through itscoilin amagnetic field.
Galvanometers were the first instruments used to detect and measure electric currents. Sensitive
galvanometers were used to detect signals from long submarine cables, and were used to discover the
electrical activity of the heart and brain. Some galvanometers used a solid pointer on a scale to show
measurements, other very sensitive types used a tiny mirror and a beam of light to provide mechanical
amplification of tiny signals. Initially a laboratory instrument relying on the Earth's own magnetic field toprovide restoring force for the pointer, galvanometers were developed into compact, rugged, sensitive
portable instruments that were essential to the development of electrotechnology. A type of galvanometer
that permanently recorded measurements was thechart recorder.The term has expanded to include uses
of the same mechanism in recording, positioning, andservomechanismequipment.
istoryThe deflection of amagnetic compassneedle by current in a wire was first described byHans Oerstedin1820. The phenomenon was studied both for its own sake and as a means of measuring electrical current.
The earliest galvanometer was reported byJohann Schweiggerat the University of Halle on 16 September
1820.Andr-Marie Amprealso contributed to its development. Early designs increased the effect of the
magnetic field due to the current by using multiple turns of wire; the instruments were at first called
"multipliers" due to this common design feature. The term "galvanometer", in common use by 1836, was
derived from the surname of Italian electricity researcherLuigi Galvani,who discovered in 1771 that electric
current could make a frog's leg jerk.
Originally the instruments relied on the Earth's magnetic field to provide the restoring force for the compass
needle; these were called"tangent" galvanometersand 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 inany orientation. The most sensitive form, the Thompson ormirror galvanometer,was improved byWilliam
Thomson(Lord Kelvin) from the early design invented in 1826 byJohann Christian Poggendorff.
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Thomson's design, which he patented in 1858, was able to detect very rapid current changes. Instead of a
compass needle, it used tiny magnets attached to a small lightweight mirror, suspended by a thread; the
deflection of a beam of light greatly magnified the deflection due to small currents. Alternatively the
deflection of the suspended magnets could be observed directly through a microscope.
The ability to quantitatively measure voltage and current allowedGeorg Ohmto formulateOhm's Law,which states that the voltage across a conductor is directly proportional to the current through it.
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-
Arsne d'ArsonvalandMarcel Deprezdevelopeda formwith 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.
Edward Westonextensively 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 in a
wristwatchbalance wheel.He developed a method of stabilizing the magnetic field of the permanent
magnet, so that the instrument would have consistent accuracy over time. He replaced the light beam and
mirror with a knife-edge pointer, which could be directly read; a mirror under the pointer and in the same
plane as the scale eliminatedparallaxerror in observation. To maintain the field strength, Weston's design
used a very narrow slot in which the coil was mounted, with a minimal air-gap and soft iron pole pieces; this
made the deflection of the instrument more linear with respect to coil current. Finally, the coil was wound on
a light-former 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 component in electricalequipment. It was known as the "portable" instrument because it was little affected by mounting position or
by transporting it from place to place. This design is almost universally used in moving-coil meters today.
Operation
D'Arsonval/Weston galvanometer movement (ca. 1900). Part of the magnet's leftpole pieceis
broken out to show the coil.
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The most familiar use is as an analog measuring instrument, often called an ammeter. It is used to
measure the direct current (flow of electric charge) through an electric circuit. The D'Arsonval/Weston form
used today is constructed with a small pivoting coil of wire 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 the angular deflection of the pointer is proportional to the current. A useful meter generally
contains provision for damping the mechanical resonance of the moving coil and pointer, so that the pointer
settles quickly to its position without oscillation.
The basic sensitivity of a meter might be, for instance, 100microamperesfull 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 calledshunts,allows a meter to
be calibrated to measure larger currents. A meter can be calibrated as a DC voltmeter if the resistance ofthe 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
aresistorin series with the meter coil. A meter can be used to readresistanceby 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.
Because the pointer of the meter is usually a small distance above the scale of the meter,parallaxerror
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 amirrored 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
anyparallaxerror has been minimized.
Today the main type of galvanometer mechanism still used is the moving coil D'Arsonval/Weston
mechanism, which is used in traditional analog meters.
Tangent galvanometer
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Tangent galvanometer made by J. H. Bunnell Co. around 1890.
A tangent galvanometer is an earlymeasuring instrumentused for the measurement ofelectric current.It
works by using acompassneedle to compare amagnetic fieldgenerated by the unknown current to themagnetic field of the Earth. It gets its name from its operating principle, the tangent law of magnetism,
which states that thetangentof the angle a compass needle makes is proportional to the ratio of the
strengths of the two perpendicular magnetic fields. It was first described byClaude Pouilletin 1837.
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 thevector sumof 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. Tangent Galvanometer] 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.
Theory
Top view of a tangent galvanometer made about 1950. The indicator needle of the compass is
perpendicular to the shorter, black magnetic needle.
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The galvanometer is oriented so that the plane of the coil is vertical and aligned along parallel to thehorizontal componentBHof the Earth's magnetic field (i.e. parallel to the local "magnetic meridian"). When
an electrical current flows through the galvanometer coil, a second magnetic fieldBis 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:
whereIis the current inamperes,nis the number of turns of the coil and ris the radius of the coil.
These two perpendicular magnetic fields addvectorially,and the compass needle points along the
direction of their resultantBH+B. The current in the coil causes the compass needle to rotate by an
angle :
From tangent law,B = BHtan , i.e.
or
orI = Ktan , whereKis 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 is 45.
When the value of is close to 0 or 90, a large percentage change in the current will only
move the needle a few degrees.[citation needed]
Geomagnetic field measurementA tangent galvanometer can also be used to measure the magnitude of the horizontal
component of thegeomagnetic field.When used in this way, a low-voltage power source,
such as a battery, is connected in series with arheostat,the galvanometer, and
anammeter.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.
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Astatic galvanometer
The astatic galvanometerwas developed byLeopoldo Nobiliin 1825.
Unlike a compass-needle galvanometer, the astatic galvanometer has two magnetic
needles parallel to each other, but with the magnetic poles reversed. The needle assembly
is suspended by a silk thread, and has no net magnetic dipole moment. It is not affected bythe earth's magnetic field. The lower needle is inside the current sensing coils and is
deflected by the magnetic field created by the passing current.
Mirror galvanometer
Thompson reflecting galvanometer.
Extremely sensitive measuring equipment once usedmirror galvanometersthat substituted
a mirror for the pointer. A beam of light reflected from the mirror acted as a long, massless
pointer. Such instruments were used as receivers for early trans-Atlantic telegraph systems,
for instance. The moving beam of light could also be used to make a record on a movingphotographic film, producing a graph of current versus time, in a device called
anoscillograph.Thestring galvanometerwas a type of mirror galvanometer so sensitive
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that it was used to make the firstelectrocardiogramof the electrical activity of the human
heart.
Ballistic galvanometer
Aballistic galvanometeris an instrument with a high moment of inertia, arranged so that its
deflection is proportional to the total charge sent through the meter's coil.
Uses
Modern closed-loop galvanometer-driven laser scanning mirror from Scanlab.
Past usesA major early use for galvanometers was for finding faults in telecommunications cables.
They were superseded in this application late in the 20th century bytime-domain
reflectometers.
Probably the largest use of galvanometers was the D'Arsonval/Weston type movement
used in analog meters in electronic equipment. Since the 1980s, galvanometer-type analog
meter movements have been displaced byanalog to digital converters(ADCs) for some
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 favor application of analog meter
movements.
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Galvanometer mechanisms were also used to position thepensin analog stripchart
recorderssuch as used inelectrocardiographs,electroencephalographsandpolygraphs
Strip chart recorders with galvanometer driven pens may have a full scale frequency
response of 100 Hz and several centimeters deflection. The writing mechanism may be a
heated tip on the needle writing on heat-sensitive paper, or a hollow ink-fed pen. In some
types the pen is continuously pressed against the paper, so the galvanometer must be
strong enough to move the pen against the friction of the paper. In other types, such as the
Rustrak recorders, the needle is only intermittently pressed against the writing medium; at
that moment, an impression is made and then the pressure is removed, allowing the needle
to move to a new position and the cycle repeats. In this case, the galvanometer need not be
especially strong
Galvanometer mechanisms were also used in exposure mechanisms in film cameras.
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-loopandopen-loop- or resonant-types
Mirrorgalvanometer systems are used as beam positioning or beam steering elements
inlaser scanning systems.For example, for material processing with high-power lasers,
mirror galvanometer are typically high power galvanometer mechanisms used with closed
loopservocontrol systems. 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 instereolithography,inlaser sintering,
inlaser engraving,inlaser beam welding,inlaser TV,inlaser displays,and in imaging
applications such asOptical Coherence Tomography(OCT) retinal scanning. Almost all of
these galvanometers are of the moving magnet type
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Open loop, or resonant mirror galvanometers, are mainly used in laser-based barcode
scanners, in some printing machines, in some imaging applications, in military applications,
and in space systems. Their non-lubricated bearings are especially of interest in
applications that require a highvacuum
A galvanometer mechanism is used for the head positioningservos inhard disk drivesandCD and DVD players. These are all of the moving coil type, in order to keep mass, and thus
access times, as low as possible
Earth's magnetic field
Computer simulation of the Earth's field in a period of normal polarity between reversals. The lines
represent magnetic field lines, blue when the field points towards the center and yellow when away. The
rotation axis of the Earth is centered and vertical. The dense clusters of lines are within the Earth's core.
Earth's magnetic field(also known as the geomagnetic field) is themagnetic fieldthat extends from
theEarth's interior to where it meets thesolar wind,a stream of charged particles emanating from theSun.
Its magnitude at the Earth's surface ranges from 25 to 65 microTesla(0.25 to 0.65Gauss). It is
approximately the field of amagnetic dipoletilted at an angle of 10 degrees with respect to the rotational
axisas if there were a bar magnet placed at that angle at the center of the Earth. However, unlike the field
of a bar magnet, Earth's field changes over time because it is generated by the motion of molten iron alloys
in the Earth'souter core(thegeodynamo).
TheNorth Magnetic Polewanders, but does so slowly enough that an ordinarycompassremains useful for
navigation. However, at random intervals, which average about several hundred thousand years,the
Earth's field reverses,which causes the north andSouth Magnetic Polesto change places with each other.
These reversals of thegeomagnetic polesleave a record in rocks that allowpaleomagnetiststo calculate
past motions of continents and ocean floors as a result ofplate tectonics.
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The region above theionosphereis called themagnetosphere,and extends several tens of thousands of
kilometers intospace.This region protects the Earth fromcosmic raysthat would otherwise strip away the
upper atmosphere, including theozone layerthat protects the earth from harmful ultraviolet radiation.
Importance
The magnetic field of the Earth deflects most of the solar wind. The charged particles in the solar wind
would strip away the ozone layer, which protects the Earth from harmfulultravioletrays.One stripping
mechanism is for gas to be caught in bubbles of magnetic field, which are ripped off by solar
winds.[4]Calculations of the loss of carbon dioxide from the atmosphere of Mars, resulting from scavenging
of ions by the solar wind, indicate that the dissipation of the magnetic field of Mars caused a near-total loss
of its atmosphere.
The study of past magnetic field of the Earth is known aspaleomagnetism.The polarity of the Earth'smagnetic field is recorded inigneous rocks,andreversals of the fieldare thus detectable as "stripes"
centered onmid-ocean ridgeswhere thesea flooris spreading, while the stability of thegeomagnetic
polesbetween reversals has allowed paleomagnetists to track the past motion of continents. Reversals
also provide the basis formagnetostratigraphy,a way ofdatingrocks and sediments.[8]The field also
magnetizes the crust, andmagnetic anomaliescan be used to search for deposits of metalores.
Humans have usedcompassesfor direction finding since the 11th century A.D. and for navigation since the
12th century. Although theNorth Magnetic Poledoes shift with time, this wandering is slow enough that a
simplecompassremains useful for navigation.
Variations in the magnetic field strength have been correlated to rainfall variation within thetropics.
Main characteristics
Description
Common coordinate systems used for representing the Earth's magnetic field.
At any location, the Earth's magnetic field can be represented by a three-dimensional vector (see figure). A
typical procedure for measuring its direction is to use a compass to determine the direction of magnetic
North. Its angle relative to true North is the declination(D) or variation. Facing magnetic North, the angle
the field makes with the horizontal is the inclination(I) or dip. The intensity(F) of the field is proportional to
the force it exerts on a magnet. Another common representation is inX(North), Y(East) andZ(Down)
coordinates.
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Intensity
The intensity of the field is often measured ingauss (G),but is generally reported innanotesla (nT),with
1 G = 100,000 nT. A nanotesla is also referred to as a gamma ().(These are the units of the so-
calledmagnetic B-field.The magnetic H-fieldhas different units, but outside of the Earth's core they are
proportional to each other.) The field ranges between approximately 25,000 and 65,000 nT (0.250.65 G).
By comparison, a strongrefrigerator magnethas a field of about 100 gauss (0.010 T).
A map of intensity contours is called an isodynamic chart. As the2010 World Magnetic Modelshows, the
intensity tends to decrease from the poles to the equator. A minimum intensity occurs over South America
while there are maxima over northern Canada, Siberia, and the coast of Antarctica south of Australia.[16]
Inclination
The inclination is given by an angle that can assume values between -90 (up) to 90 (down). In the
northern hemisphere, the field points downwards. It is straight down at theNorth Magnetic Poleand rotates
upwards as the latitude decreases until it is horizontal (0) at the magnetic equator. It continues to rotate
upwards until it is straight up at theSouth Magnetic Pole.Inclination can be measured with adip circle.
An isoclinic chart(map of inclination contours) for the Earth's magnetic field is shownbelow.
Declination
Declination is positive for an eastward deviation of the field relative to true north. It can be estimated by
comparing the magnetic north/south heading on a compass with the direction of acelestial pole.Maps
typically include information on the declination as an angle or a small diagram showing the relationship
between magnetic north and true north. Information on declination for a region can be represented by a
chart with isogonic lines (contour lines with each line representing a fixed declination).
Geographical variation
Components of the Earth's magnetic field at the surface from theWorld Magnetic Modelfor 2010.
Intensity
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inclination
Declination
Dipolar approximation
The variation between magnetic north (Nm) and "true" north (Ng).
Near the surface of the Earth, its magnetic field can be closely approximated by the field of amagnetic
dipolepositioned at the center of the Earth and tilted at an angle of about 10 with respect to therotational
axisof the Earth. The dipole is roughly equivalent to a powerful barmagnet,with its south pole pointing
towards thegeomagnetic North Pole.This may seem surprising, but the north pole of a magnet is so
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defined because, if allowed to rotate freely, it points roughly northward (in the geographic sense). Since the
north pole of a magnet attracts the south poles of other magnets and repels the north poles, it must be
attracted to the south pole of Earth's magnet. The dipolar field accounts for 8090% of the field in most
locations.
Magnetic poles
The movement of Earth's North Magnetic Pole across the Canadian arctic, 18312007.
The positions of the magnetic poles can be defined in at least two ways: locally or globally.[18]
One way to define a pole is as a point where themagnetic fieldis vertical.[19]This can be determined by
measuring the inclination, as described above. The inclination of the Earth's field is 90 (upwards) at
theNorth Magnetic Poleand -90(downwards) at theSouth Magnetic Pole.The two poles wander
independently of each other and are not directly opposite each other on the globe. They can migrate
rapidly: movements of up to 40 kilometres (25 mi) per year have been observed for the North Magnetic
Pole. Over the last 180 years, the North Magnetic Pole has been migrating northwestward, from Cape
Adelaide in theBoothia Peninsulain 1831 to 600 kilometres (370 mi) fromResolute Bayin
2001.[20]The magnetic equatoris the line where the inclination is zero (the magnetic field is horizontal).
The global definition of the Earth's field is based on a mathematical model. If a line is drawn through the
center of the Earth, parallel to the moment of the best-fitting magnetic dipole, the two positions where it
intersects the Earth's surface are called the North and Southgeomagnetic poles.If the Earth's magnetic
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field were perfectly dipolar, the geomagnetic poles and magnetic dip poles would coincide and compasses
would point towards them. However, the Earth's field has a significantnon-dipolarcontribution, so the poles
do not coincide and compasses do not generally point at either.
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