Question

Briefly explain the elements required to specify the earths magnetic field at a given place

Answer 1

Three element required to specify the earth's magnetic field at a given place is given as follows
 1. Declination(θ) 2.inclination or dip angle UPB(δ) and 3. horizontal component of the field.

 Starting from the geographical meridian we drow the magnetic meridian at an angle QPU (declination). In the magnetic meridian we  drow the horizontal direction
specifying magnetic north.Magnetic field is at an angle delta from this direction.The horizontal component BH and the total field B are related as
BH=Bcosδ⇒B=BHcosδ.
Thus , from the knowledge of the three element, both the magnitude and direction of the earth's magnetic field can be obtained.

Answer 2

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The three independent quantities used to specify the earth’s magnetic field are the following.

1) Magnetic declination: It is the angle between the geographic north and the magnetic north at a place.

Note: $<u>If the compass deflects right of geographic north, then declination is positive and if deflection is towards the left of geographic north then it is a negative declination</u>$

2) Angle of Dip: It is the angle between the horizontal plane and the magnetic axis, as observed in the compass.

3) The horizontal component of the earth’s magnetic field