THE EARTH'S MAGNETISM. —Dip of the Needle

Magnetic Declination: The horizontal angle between magnetic meridian and true meridian is known as ‘magnetic declination’.

  When north end of the magnetic needle is pointed towards the west side of the true meridian the term ‘Declination West’ (θ W) (Fig. 7.a).

  When north end of the magnetic needle is pointed towards the east side of the true meridian the term ‘Declination East’ (θ E) (Fig. 7.b).

Isogonic and agonic lines:

  Lines passing through equal declination are known as ‘isogonic’ lines.

  The lines passing through zero declination is said to be the ‘aogonic’ line (Fig. 8).

Variation of Magnetic Declination: The magnetic declination at a place is not constant. It varies due to the following reasons:

a). Secular Variation: The magnetic meridian behaves like a pendulum with respect to the true meridian. After every 100 years or so; it swings from one direction to the opposite direction, and hence the declination varies. This variation is known as ‘secular variation’.

b). Annual Variation: The magnetic declination varies due to the rotation of the earth, with its axis inclined, in an elliptical path around the sun during the year. This variation is known as ‘annual variation’. The amount of variation is about 1 to 2 minutes.

c). Diurnal Variation: The magnetic declination varies due to the rotation of the earth, with its own axis in 24 hours. This variation is known as ‘diurnal variation’. The amount of variation is found to be about 3 to 12 minutes.

d). Irregular Variation: The magnetic declination is found to vary suddenly due to some natural causes, such as earthquake, volcanic eruptions and so on. This variation is known as ‘irregular variation’.

Dip of the magnetic needle: If a needle perfect balanced before magnetisations, it does not remain balanced position after it is magnetised. This is due to magnetic influence of earth. The needle is found to be inclined toward the pole. This inclination of the needle with the horizontal is known as ‘dip of the magnetic needle’.

  It is found that the north end of the needle is deflected downwards in the northern hemisphere and that its south end deflected downwards in the southern hemisphere. The needle is just horizontal at the equator. To balance dip of the needle, a rider (brass or silver coil) it provided along with it. The rider is placed over the needle at a suitable position to make it horizontal.

Local attraction: A magnetic needle indicates the north direction when freely suspended or pivoted. But the needle comes near some magnetic substances, such as iron ore, steel structure, electric cable conveying current: etc. it is found to be deflected from its true direction, and dose not show the actual north. This disturbing influence of magnetic substance is known as the ‘local attraction.    

  To detect the present of local attraction, the fore and back bearing of a line should be taken. If the difference fore and back bearings of the line is exactly 180⁰, then there is no local attraction.

  To compensate of local attraction, the amount of error found out and is equally distributed between fore and back bearings of the line.

  For example, consider a case when

Observed FB of line AB = 70⁰30’

Observed BB of line AB = 230⁰0’

Calculated BB of line AB = 70⁰30’ + 180⁰ = 250⁰30’

Corrected BB of line AB = ½ (230⁰0’+250⁰30’) =240⁰15’

Hence Corrected FB of line AB = 240⁰15’ -180⁰ = 60⁰15’

Method of application of correction

a). First method: The interion angles of a traverse are calculated from the observed bearings. Then an angular check is applied. The sum of interion angles should be equal to (2n-4) x 90⁰ (n being the number of sides of the traverse. If it is not so, total error equally distributed among all the angles of the traverse.

  Then starting from the unaffected line, the bearings of lines may be corrected by using the corrected interior angles. This method is very laborious and is not generally employed.

b). Second method: In this method interior angles are not calculated. From the given table, the unaffected line is first detected. Then, commencing from the unaffected line, the bearings of the other affected lines are connected by finding the amount of correction at each station.

  This is an easy method, and one which is generally employed.

  If the all lines of a traverse are found to be affected by local attraction, the line with minimum error is identified. The FB and BB of this line are adjusted by distributing the error equally. Then, starting from the this adjusted line, the fore and back bearings of other lines are corrected.

  

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