ACKNOWLEDGEMENT

My first experience of project has been successfully completed, thanks to the support staff of many friends & colleagues with gratitude. We wish to acknowledge all of them. However, we wish to make special mention of the following.

First of all we are thankful of our project guide – DR.RAJWANT KALSI under whose guideline we were able to complete our project. We are wholeheartedly thankful to them for giving us their valuable time & attention & for giving us the opportunity to work on such an interesting topic.

We would thank to AMIT SIR for his support, ideas and guidance as without him this project would have been a distant reality.

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contentsw

TOPIC PAGE NUMBERGALVANOMETER 3INVENTION OF GALVANOMETER

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TYPES OF GALVANOMETER

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ELEMENTS OF EARTH’S MAGNETISM

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LAW OF TANGENT IN MAGNETOSTATICS

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OTHER COMPONENTS OF EXPERIMENTAL SETUP

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EXPERIMENT 17QUESTIONARRIE 23

Galvanometer

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Galvanometer is an instrument used to indicate the presence, direction, or strength of a small electric current. The typical galvanometer is a sensitive laboratory instrument used mainly to detect and compare currents.

Galvanometer can be categorized as a type of an ammeter. A moving-coil mechanism similar to that used in a galvanometer is used in some ammeters. Like the galvanometer, these instruments measure the strength of a current but they can handle a stronger current; unlike the galvanometer, they cannot indicate the current's direction.

Its design is based on the premise that a magnetic needle moves in the presence of electric current in a conductor placed near the galvanometer. When the current flows through the conductor, the needle turns parallel to the line of induction about the conductor. The North Pole points in the induction flow’s direction and how much the needle turns is based on the electrical current’s overall strength.

Moreover, galvanometer is an important practitioner tool in research labs. By means of galvanometer students learn about basic properties of current, its flow and various resistances and their effects on flow of current. Galvanometers also help to design a special safety guideline for a lab in which any sort of electrical work is going on.

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INVENTION OF GALVANOMETER

Driving its name for Luigi Galvani, who is said to have designed the first prototype of the instrument, the galvanometer is more usually associated with William Thomason, who experimented with the unit to expand the number of common uses for the device.

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 electrical current.

The earliest galvanometer was reported by Johann Schweigger at the University of Halle on 16 September 1820. André-Marie Ampère is credited with inventing the galvanometer in 1824. The first type was a tangent galvanometer, which had a compass surrounded by a coil of wire. The tangent of the angle the needle was moved on was proportional to the current’s strength in the coil being measured. These original galvanometers were later improved upon by putting the compass in the center of a more precisely measured circle, which significantly improved their accuracy. Even later, the compass was replaced with a meter and leveling screws were then added to the devices.

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 in a wristwatch balance 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 eliminated parallax error in observation.

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TYPES OF GALVANOMETER

REFLECTING GALVANOMETERS

o The early tangential galvanometers were limited in that their needles had to be very short to minimize the effects of the earth’s magnetic field and other errors on the device’s measurements. The short needle made it hard to measure small changes in electrical current. The reflecting galvanometer solved this problem as it replaced the needle with a beam of light that was reflected off of a mirror onto a scale placed three feet from the device. These permitted small current changes to be measured.

STRING GALVANOMETERS o Suspending the device’s needle at the end of a string further improved

the sensitivity of galvanometer measurement. A static models used two needles mounted in parallel with opposite poles to neutralize the effect of the earth’s magnetic field.

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MIRROR GALVANOMETERS  

o 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 moving photographic film, producing a graph of current versus time, in a device called an oscillograph.

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BALLISTIC GALVANOMETER

o Ballistic galvanometer is a type of mirror galvanometer. It can be either of the moving coil or moving magnet type, but unlike a current-measuring galvanometer, the moving part has a large moment of inertia, thus giving it a low oscillation period. It is really an integrator measuring the quantity of charge discharged through it.

This is a moving-coil ballistic galvanometer with a very small restoring torque used for measuring magnetic flux. It is calibrated in maxwells, an early unit of magnetic flux. It is used with an exploring coil in a closed circuit. The meter is set by mechanical means to zero while the coil is in the field to be measured. The coil is then removed from the field, causing a deflection on the meter, which yields the change of magnetic flux in the coil.

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TANGENT GALVANOMETER

Tangent galvanometer was made by J.H Bunnell Co. around 1890.

A galvanometer in which a small compass is mounted horizontally in the center of a large vertical coil of wire; the current through the coil is proportional to the tangent of the angle of deflection of the compass needle from its normal position parallel to the magnetic field of the earth. 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 operating principle, the tangent law of magnetism, which states that the tangent of the angle a compass needle makes is proportional to the ratio of the strengths of the two perpendicular magnetic fields.

A single coil tangent galvanometer is the simplest instrument which affords a method for the measurement of electric current. The construction of the instrument is based on the fact that current through circular coil produces a magnetic field at the centre of the coil in a direction perpendicular to the plane of the coil, and the field is proportional to the current. The instrument is based on tangent law.Adjustments - Before using the instrument, the following adjustments have to be made.(i) The instrument must be leveled with the help of leveling screws so that the base is horizontal and hence the coil is vertical. The horizontality can be checked using a spirit level. The compass box must be adjusted such that the magnet swings freely and is parallel to its image.(ii) The plane of the coil must be set in the magnetic meridian. This can be brought about by first rotating the compass box till the 90°-90° line is in the plane of the coil, and then rotating the coil, as a whole, so that the pointer reads 0°-0°.

Fig (a) shows a form of single coil tangent galvanometer. It consists essentially of a circular coil of insulated copper wire, having a number of turns.The coil is wound over a circular frame of wood or ebonite. The, frame is fixed in a vertical plane on a horizontal base provided with leveling screws. The frame is so arranged that the coil, as a whole, can be rotated about a

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vertical axis passing through the centre of the coil. The ends of the coil are connected to binding screws provided on the base. A deflection magnetometer compass box is mounted horizontally at the centre of the coil so that its pivot coincides with the centre of the coil.

Fig(a) tangent galvanometer

Elements of Earth's Magnetism

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A meridian (or line of longitude) is an imaginary arc on the Earth's surface from the North Pole to the South Pole that connects all locations running along it with a given longitude. The position of a point on the meridian is given by the latitude.

GEOGRAPHIC MERIDIAN : A geographic meridian runs straight north

and south, from the North Pole to the South Pole.

MAGNETIC MERIDIAN: A line on the earth’s surface, passing in the

direction of the horizontal component of the earth’s magnetic field is called magnetic meridian.

The physical quantities which completely describe earth’s magnetic field are called elements of earth's magnetism. These are declination, dip or inclination and horizontal component of earth's magnetic field.

DECLINATION (Q):  Declination at a place is the angle between the

geographic meridian and the magnetic meridian at that place. It is denoted by q. 

The angle of declination varies from place to place on the earth's surface. The declination is helpful is steering ships in the right direction with the help of mariner's compass.

DIP OR INCLINATION: Dip or inclination at a place is the angle between

the direction of intensity of earth's total magnetic field (R) and the horizontal direction in the magnetic meridian at that place. It is denoted by d. At the earth's magnetic poles, the magnetic field of earth is perpendicular to earth's surface. Therefore, the value of dip is 90o at earth's magnetic poles: The dip needle

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becomes vertical at these locations. At the magnetic equator, d = 0o so that dip needle becomes horizontal.

HORIZONTAL COMPONENT OF EARTH'S MAGNETIC FIELD: It is the component of earth's total magnetic field along horizontal direction in the magnetic meridian. It is denoted by H.Once we know the values of declination (q), dip (d) and horizontal component (H) of earth's magnetic field at a place, we can specify the strength and direction of earth’s magnetic held at that place.

Law of tangents in magnetostatics

To understand law of tangents, let’s consider that a magnetic dipole, having dipole moment µ, is placed under the influence of two magnetic fields (B and BH) which are perpendicular to each other. The magnetic dipole lies in the plane subtended by two magnetic fields and makes an angle of θ with the magnetic field BH. The magnetic dipole will suffer torque from each of the applied magnetic fields which are given to be:

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τH = µ BHsinθ [Balancing of torque by both magnetic field of earth as well as that

produced in a circular loop.]

τ = µ Bsin(90-θ)= µ Bcosθ

Each of the torques, expressed above, have tendency to rotate the magnetic dipole so as to align its magnetic moment along the direction of field. The torques will have opposing action and counterbalance each other at some orientation of the dipole and this can be defined by the inclination θ with the magnetic field BH. Hence we will have:

τ = τH

µB cos θ=µBH sin θ

B=BHtan θ

In the Stewart and Gee Tangent galvanometer, the magnetic field B, due to current carrying vertical coil, is produced perpendicular to the horizontal component of earth’s magnetic field (BH).

Other Components of experimental set up

AMMETER:

Ammeter is an instrument for the measurement of electric current. The operating principle of an ammeter depends on the nature of the current to be measured and the accuracy required. The unit of current, the ampere, is the base unit on which

rests the International System (SI) definitions of all the electrical units. Instruments used to measure smaller currents, in the milliampere or microampere range, are designated as milliammeters or microammeters.

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The measurement of current in terms of the voltage that appears across a resistive shunt through which the current passes has become the most common basis for ammeters, primarily because of the very wide range of current measurement that it makes possible, and more recently through its compatibility with digital techniques. 

RHEOSTAT:

The adjustable resistor used in applications that require the adjustment of current or the varying of resistance in an electric circuit is called Rheostat. The symbol for

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a rheostat is a resistor symbol with an arrow diagonally across it. The rheostat can adjust generator characteristics, dim lights, and start or control the speed of motors. Its resistance element can be a metal wire or ribbon, carbon, or a conducting liquid, depending on the application. Rheostats are made with a variety of resistance & current carrying capacity so as to satisfy every need. They are wound with heavily oxidized resistance wire upon a porcelain tube dia 44 mm aprox. The tube is carrying upon Bakelite rider and the heavy duty sliding contact is of the phosphor bronze with brass rod.

REVERSING KEY:

Reversing key is a technical equipment used in circuits, where a rapid changeover of polarity is required. The switch comprises a rotatable spring loaded beam of insulating plastic with two brushes each connected to terminals. The brushes and contact strips are arranged on the beam and base respectively so that when beam is turned from one extreme to other, the polarity of connections is reversed.

BATTERY ELIMINATOR:

A device that operates from power line to supply current and voltage to a circuit designed to be operated by a battery is called battery eliminator. It converts the Source AC Current to DC Voltage that may be used by other device. The First commercial battery eliminators were produced by Edward S. Rogers, Sr. in the year 1925. Mainly battery eliminators are used for small ranges of DC

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voltage between 1.5V to 12V.

MAGNETOMETER:

Magnetometers are scientific instruments that are used to measure the direction and strength of magnetic fields in the location that the device is located. They are used both on Earth as well on space exploration missions due to the significant variations that are found in magnetic fields based on the nature of the environment, interaction of charged particles from the sun.

Magnetometers are not just used to measure the local strength of a magnetic field,

however, as they can also determine the direction and orientation. Magnetism varies from place to place and differences in Earth's magnetic field (the magnetosphere) can be caused by the differing nature of rocks and the interaction between charged particles from the Sun and the magnetosphere.

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EXPERIMENTAIM: To study the variation of magnetic field with the distance along the axis of circular coil carrying current using Stewart and Gee’s tangent galvanometer and to draw the graph between the distance from the centre and tangent of angle of deflection.

APPARATUS:

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Stewart and Gee’s tangent galvanometer, Battery eliminator, Rheostat, Ammeter, One way and reverse key and Connecting wires.

THEORY:

Set up:

Stewart and Gee’s tangent galvanometer set up consists of a circular frame (made of wood or brass or non-magnetic silver aluminum alloy) on which are wounded a fixed number of turns of insulated copper wire. In this instrument, the circular frame has three coils of 2, 50and 500 turns. The coil is fixed on a horizontal base with its plane vertical and free ends of the wire are connected to two terminals T1 and T2 provided at the base. A deflection magnetometer is fitted on two pillars V1 and V2 provided on the horizontal board in such a manner that it can slide along the horizontal direction with the center of the needle always lying on the axis passing through the center of plane of coil. The distance of the needle from the center of coil can be read on the scale provided on the magnetometer arms with reference to fixed marks A and B on the vertical pillars.

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MAGNETIC FIELD DUE TO CURRENT CARRYING COIL:

The intensity of magnetic field at a point lying on the axis of a circular coil is given by

B =

Where, n = number of turns in the coil

r = radius of the coil

i = current in ampere flowing in the coil

x = distance of the point from the center of the coil

The units of B are Wb/m2 or Tesla.

The direction of the magnetic field at any point P on the axis is along OP (where O is center of coil) if the current in the coil is in counterclockwise direction and along PO if the sense of current in the coil is clockwise. The magnetic field intensity is maximum at the center of coil and decreases as we move away from the center of coil. The variation of the magnetic field as a function of distance from center of coil is shown in figure. The curve is first concave towards O but the curvature decreases and changes sign at points P and Q and afterwards become convex towards O. The points P and Q, which are inflexion points of curve, correspond to distance x=±a/2 and distance of separation between these two points is equal to the radius of coil.

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DIAGRAM:

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PROCEDURE: Place the instrument on table such that the arms of magnetometer lie roughly

along east west direction and the magnet needle lies at the centre of vertical coil. The coil is thus set roughly in magnetic meridian. Rotate the compass ox so that the pointer lies on the 0-0 line.

Connect the galvanometer to the battery eliminator, a one way key and an ammeter through a reversing key as shown in the circuit diagram.

Adjust the value of the current so that magnetometer gives a deflection of order 60-70 degrees. Reverse the current and again note the deflection.

Take the reading of it at x=0 for direct and reverse direction of current.

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Now slide the magnetometer by 2cm in left and right direction and take the direct and reverse readings.

OBSERVATIONS:

S.

No

.

Dis

tan

ce m

oved

(cm

)(x)

Deflection on left

side

Mean

(in degree)

Deflection on right

side

Mean

(in degree)

Direct

Current

Reverse

Current

Direct

Current

Reverse

Current

θ1 θ2 θ3 θ4 θ1 θ2 θ3 θ4

1.

2.

3.

4.

5.

6.

7.

8.

0

2

4

6

8

10

12

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CALCULATIONS:

x (in LEFT SIDE RIGHT SIDE

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c.m.) θ (in degree) tan θ θ (in degree) tan θ

0

2

4

6

8

10

12

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PRECAUTIONS: There should be no magnetic substance or current carrying conductor near

the galvanometer.

The plane of the coil must be set in magnetic meridian.

To avoid error the current should be kept constant and should be reversed for all directions.

Eye should be placed in such a way that the pointer covers its image in the mirror below.

RESULT:The magnetic field is highest at centre of circular coil and decreases as we move to left or right symmetrically.

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SOURCES OF ERROR: Instruments used might be faulty. Base might not be horizontal (which might be checked using spirit level). Plane of coil might not be set in magnetic meridian.

QUESTIONAIRE:

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HOW DOES THE FIELD VARY ALONG THE AXIS OF THE COIL?Magnetic field B at a distance x from the centre is given as

Where n is the number of turns and I is the current flowing through the coil.

WHAT IS MAGNETIC MERIDIAN?A line on the earth’s surface, passing in the direction of the horizontal component of the earth’s magnetic field is called magnetic meridian.

WHY IS IT NECESSARY TO SET TH PLANE OF COIL IN MAGNETIC MERIDIAN?To make the magnetic field B produced by circular coil at right anhles to the horizontal components of earth’s magnetic field.

HOW THE ERROR IN THE MEASUREMENT OF DEFLECTION IS ELIMINATED IF THE PLACE OF COIL IS NOT EXACTLY IN MAGNETIC MERIDIAN?By passing current in one direction and then in reverse direction and taking the mean of these four readings, we get deflection free from error.

WHAT IS TANGENT LAW?B=BH tan θ where is the angle made by magnetic needle with horizontal component of earth’s magnetic field BH .B and BH are two mutually perpendicular uniform magnetic fields.

WHAT PRECAUTIONS SHOULD BE TAKEN IN THE EXPERIMENT?There should be no magnets, magnetic substance or current carrying conductors near the coil.Also the current should remain constant for all observations.

WHAT CAN BE DONE TO AVOID PARALLAX ERROR?To avoid parallax error, the eye should be placed in such a way that the pointer covers its image in the mirror below.

WHY 50 TURNS OF COPPER WIRE IS USED?

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We can use any amount of copper windings but using more turns means greater magnetic field and with that we have to adjust the distance between the two consecutive readings.

WHAT IS A GALVANOMETER?

Galvanometer is an instrument used to indicate the presence, direction, or strength of a small electric current. The typical galvanometer is a sensitive laboratory instrument used mainly to detect and compare currents.

WHAT IS DIP OR INCLINATION?

Dip or inclination at a place is the angle between the direction of intensity of earth's total magnetic field (R) and the horizontal direction in the magnetic meridian at that place. It is denoted by d. 

WHO INVENTED TANGENT GALVANOMETER?Tangent galvanometer was made by J.H Bunnell Co. around 1890.

WHAT ARE THE ADJUSTMENTS TO BE MADE IN TANGENT GALVANOMETER?The instrument must be leveled with the help of leveling screws so that the base is horizontal and hence the coil is vertical.Also, the plane of the coil must be set in the magnetic meridian.

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