galvanometer and earths magnetic field by shubham wasule 7566220325

8/13/2019 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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galvanometer and earths magnetic field by shubham wasule 7566220325

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