Magnetic elements of earth at a place are the quantities which describe completely in magnitude as well as direction the magnetic field of earth at that place.
Following are the three magnetic elements of earth
magnetic declination (θ)
magnetic inclination or magnetic dip (δ)
horizontal component (H)
Magnetic declination when a magnet is suspended freely by an unsung silk thread the ends of the magnet do not point out exactly along geographic north south direction the line joining the north and south poles of a freely suspended magnet is called magnetic axis. The axis about which earth rotates is the geographic axis.
The small gale between magnetic axis and geographic axis at a place is defined as the magnetic declination at the place. It is represented by θ.
A vertical plane passing through N – S lines of a freely suspended magnet is called magnetic meridian and the vertical plane passing through the geographic north-south direction is called geographic meridian. We therefore also define magnetic declination at a place as the angel between magnetic meridian and geographic meridian at that place.
The magnetic axis and CD represents the geographic axis. The vertical plane CD AB is ht magnetic meridian and the vertical plane CD AB is the geographic meridian by definition lce C = / BAB = θ is the magnetic declination.
Magnetic declination arises because the axis of the fictitious bar magnet responsible for earth’s magnetism does not coincide with the axis of rotation of earth (which is the line joining the geographic north N and geographic south S) infect the imaginary magnet lies at an angle of about 20 west of the polar axis as this is the order of magnetic declination at the place.
Note that earth magnetic poles are not at directly opposite positions on globe. Current magnetic south Is farther from geographic south than magnetic north is from geographic north.
Infact the magnetic field of earth varies with position and also with time for example in a span of 240 years from 1580 to 1820 AD, the magnetic declination at condom has been found to changes by 3.5 suggesting that magnetic poles of earth change position with time.
The magnetic declination in India is rather small. At Delhi declination is only 041 east and at Mumbai the declination is 0’58’ west. It means that the direction of geographic north is given quite accurately by th compass needle.
Magnetic dip or magnetic inclination at a place is defined as the angle which the direction of total strength of earth magnetic field makes with a horizontal line in magnetic meridian. It is the angle by which to total intensity of earth’s magnetic field dips or cones up out of the horizontal plane. It is represented by. The value of magnetic dip at a place can be measure using an instrument called dip circle.
A magnetic needle NS is pivoted on a horizontal axle at the centre of a circular vertical scale V. the ends of magnetic needle are free to move over this scale graduated in degrees marked O – O in the horizontal direction and 90 – 90 in the vertical direction. The needle and the scale are enclosed in a rectangular box B with glass windows the box is mounted on a vertical pillar p on horizontal base HB provided with leveling screws L & L A circular scale C (0 to 360) is engraved on the HB. The box can be rotated over the horizontal base and its position can be read on the circular scale C with the help of a venire scale (not shown).
To use the dip circle
Adjust the leveling screws till the base is horizontal and scale V inside the box is vertical.
Rotate the box B till the ends of magnetic needle NS read 90 – 90 on the vertical scale.
Obviously the needle in this position is aligned with the vertical component of earth’s field the horizontal component of earth field being perpendicular to this plane does not affect the needle.
The reading of the venire is noted on horizontal circular scale.
Rotate the box exactly through 90 from this position. The plane of vertical scale V is now in mantic meridian. The total intensity R of earth’s magnetic field is in the same plane and hence magnetic needle rests along R angle which R makes with the horizontal direction in magnetic meridian is the angle of dip. Hence the readings of the ends of magnetic needle on the vertical scale given us directly the angle of dip
At different paves on the surface of earth δ is different. For example at the two magnetic poles the magnetic needle rests vertically. Therefore, angle of dip at the two poles is 90 obviously in the northern hemisphere where South Pole of flcitionus magnet lies the north pole of the mounted magnetic needle will dip down and vice-versa. On the magnetic equator the angle of dip is zero as the needle would rest horizontally the magnetic equator. At other places the value of δ lies between 0 and 90. The dip angle at Delhi is about 42 below the horizontal.
Note the magnetic declination θ is determined by a compass needle free to rotate in the horizontal plane about the vertical axis. And the magnetic dip angle δ is measured by allowing the compass needle to rotate freely in the vertical plane of magnetic meridian about the horizontal axis.
Horizontal component is the component of total intensity of earth’s magnetic field in the horizontal direction in magnetic meridian. It is represented by H.
In AK represents the total intensity of earth’s magnetic field / BAK = δ. The resultant intensity R is resolved into two rectangular components:
Horizontal component along AB is
AL = H = R cos δ
Vertical component along AD is
AM = V = R sin δ
Square (23) and (24) and add
H2 + V2 = R2 (cos2 δ + sin2δ) R2
R = H2 + V2
Dividing (24) by (23) we get
Tan δ = V/H
R sin δ / R cos δ = V / H or
The value of horizontal component H = R cos δ is different at different places. At the magnetic poles δ = 90
∴ H = R cos 90° = zero
At the magnetic equator δ = 0
∴ H = R cos θ = R
Horizontal component (H) can be measured using a vibration magnetometer and a deflection magnetometer.
The value of H at a place on the surface of earth is of the order of 3.2 X 10-5 tesla.
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