# Magnetic Maps and Neutral Points

## Magnetic Maps and Neutral Points

Magnetic Maps: Magnetic map is obtained by drawing lines on the surface of earth, which passes through different places having same magnetic elements. Magnetic maps i.e.., declination, dip and horizontal component over the earth vary in magnitude from place to place. It is found that many places have the same value of magnetic elements.

The lines are drawn joining all places on the earth having same value of a magnetic element. These lines form magnetic map.

a) Isogonic lines: These are the lines on the magnetic map joining the places of equal declination.

b) Agonic line: The line which passes through places having zero declination is called agonic line.

c) Isoclinic lines: The line joining the points of equal dip or inclination.

d) Aclinic line: The line joining places of zero dip is called aclinic line or magnetic equator.

e) Isodynamic lines: The lines joining the points or places having the same value of horizontal component of earth’s magnetic field are called isodynamic lines.

Neutral Points: A neutral point is a point at which the resultant magnetic field is zero. In general the neutral point is obtained when horizontal component of earth’s field is balanced by the field produced by the magnet.

When North Pole of a bar magnet is placed towards South Pole of the earth, then neutral point is obtained on axial line:

$$B=\frac{{{\mu }{0}}}{4\pi }\left( \frac{2Mr}{{{\left( {{r}^{2}}-{{l}^{2}} \right)}^{2}}} \right)=H;\,If\,r>>l,\,then\,B=\frac{{{\mu }{0}}}{4\pi }\left( \frac{2M}{{{r}^{3}}} \right)=H$$,

When North Pole of a bar magnet is placed towards North Pole of the earth, then neutral point is obtained on equatorial line:

$$B=\frac{{{\mu }_{0}}}{4\pi }\left( \frac{2Mr}{{{\left( {{r}^{2}}+{{l}^{2}} \right)}^{2}}} \right)=H;\,If\,r>>l,\,\,then\,\,B=\frac{{{\mu }_{0}}}{4\pi }\left( \frac{2M}{{{r}^{3}}} \right)=H$$.