`color{blue} ✍️` Consider a point on the earth’s surface. At such a point, the direction of the longitude circle determines the geographic north-south direction, the line of longitude towards the north pole being the direction of true north.
`color{blue} ✍️` The vertical plane containing the longitude circle and the axis of rotation of the earth is called the geographic meridian. In a similar way, one can define magnetic meridian of a place as the vertical plane which passes through the imaginary line joining the magnetic north and the south poles.
`color {blue}{➢➢}` This plane would intersect the surface of the earth in a longitude like circle. A magnetic needle, which is free to swing horizontally, would then lie in the magnetic meridian and the north pole of the needle would point towards the magnetic north pole.
`color {blue}{➢➢}` Since the line joining the magnetic poles is titled with respect to the geographic axis of the earth, the magnetic meridian at a point makes angle with the geographic meridian. This, then, is the angle between the true geographic north and the north shown by a compass needle. This angle is called the magnetic declination or simply declination (Fig. 5.9).
`color{blue} ✍️` The declination is greater at higher latitudes and smaller near the equator. The declination in India is small, it being `color{blue}(0^(0) 41′ E)` at Delhi and `color{blue}(0^(0) 58′ W)` at Mumbai. Thus, at both these places a magnetic needle shows the true north quite accurately.
`color{blue} ✍️` There is one more quantity of interest. If a magnetic needle is perfectly balanced about a horizontal axis so that it can swing in a plane of the magnetic meridian, the needle would make an angle with the horizontal (Fig. 5.10).
`color{blue} ✍️` This is known as the angle of dip (also known as inclination). Thus, dip is the angle that the total magnetic field `color{blue}(B_E)` of the earth makes with the surface of the earth. Figure 5.11 shows the magnetic meridian plane at a point `color{blue}(P)` on the surface of the earth. The plane is a section through the earth. The total magnetic field at `color{blue}(P)` can be resolved into a horizontal component `color{blue}(H_E)` and a vertical component `color{blue}(Z_E)`. The angle that `color{blue}(B_E)` makes with `color{blue}(H_E)` is the angle of dip, `color{blue}(I)`.
`color {blue}{➢➢}` In most of the northern hemisphere, the north pole of the dip needle tilts downwards. Likewise in most of the southern hemisphere, the south pole of the dip needle tilts downwards.
`color{blue} ✍️` To describe the magnetic field of the earth at a point on its surface, we need to specify three quantities, viz., the declination `color{blue}(D)`, the angle of dip or the inclination `color{blue}(I)` and the horizontal component of the earth’s field `color{blue}(H_E)`. These are known as the element of the earth’s magnetic field.
`color{blue} ✍️` Representing the verticle component by `color{blue}(Z_E)` we have
`color{blue}(Z_E B_E sin I)`
......................(5.10(a))
`color{blue}(H_E = B_E cosI)`
................(5.10 (b))
`color {blue}{➢➢}`which gives,
`color{blue}(tan I = (Z_E)/(H_E))`
.....................(5.10(c))
`color{blue} ✍️` Consider a point on the earth’s surface. At such a point, the direction of the longitude circle determines the geographic north-south direction, the line of longitude towards the north pole being the direction of true north.
`color{blue} ✍️` The vertical plane containing the longitude circle and the axis of rotation of the earth is called the geographic meridian. In a similar way, one can define magnetic meridian of a place as the vertical plane which passes through the imaginary line joining the magnetic north and the south poles.
`color {blue}{➢➢}` This plane would intersect the surface of the earth in a longitude like circle. A magnetic needle, which is free to swing horizontally, would then lie in the magnetic meridian and the north pole of the needle would point towards the magnetic north pole.
`color {blue}{➢➢}` Since the line joining the magnetic poles is titled with respect to the geographic axis of the earth, the magnetic meridian at a point makes angle with the geographic meridian. This, then, is the angle between the true geographic north and the north shown by a compass needle. This angle is called the magnetic declination or simply declination (Fig. 5.9).
`color{blue} ✍️` The declination is greater at higher latitudes and smaller near the equator. The declination in India is small, it being `color{blue}(0^(0) 41′ E)` at Delhi and `color{blue}(0^(0) 58′ W)` at Mumbai. Thus, at both these places a magnetic needle shows the true north quite accurately.
`color{blue} ✍️` There is one more quantity of interest. If a magnetic needle is perfectly balanced about a horizontal axis so that it can swing in a plane of the magnetic meridian, the needle would make an angle with the horizontal (Fig. 5.10).
`color{blue} ✍️` This is known as the angle of dip (also known as inclination). Thus, dip is the angle that the total magnetic field `color{blue}(B_E)` of the earth makes with the surface of the earth. Figure 5.11 shows the magnetic meridian plane at a point `color{blue}(P)` on the surface of the earth. The plane is a section through the earth. The total magnetic field at `color{blue}(P)` can be resolved into a horizontal component `color{blue}(H_E)` and a vertical component `color{blue}(Z_E)`. The angle that `color{blue}(B_E)` makes with `color{blue}(H_E)` is the angle of dip, `color{blue}(I)`.
`color {blue}{➢➢}` In most of the northern hemisphere, the north pole of the dip needle tilts downwards. Likewise in most of the southern hemisphere, the south pole of the dip needle tilts downwards.
`color{blue} ✍️` To describe the magnetic field of the earth at a point on its surface, we need to specify three quantities, viz., the declination `color{blue}(D)`, the angle of dip or the inclination `color{blue}(I)` and the horizontal component of the earth’s field `color{blue}(H_E)`. These are known as the element of the earth’s magnetic field.
`color{blue} ✍️` Representing the verticle component by `color{blue}(Z_E)` we have
`color{blue}(Z_E B_E sin I)`
......................(5.10(a))
`color{blue}(H_E = B_E cosI)`
................(5.10 (b))
`color {blue}{➢➢}`which gives,
`color{blue}(tan I = (Z_E)/(H_E))`
.....................(5.10(c))