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www.GOALias.blogspot.com0º41′ E at Delhi and 0º58′ W at Mumbai. Thus, at both these places amagnetic needle shows the true north quite accurately.There is one more quantity of interest. If a magnetic needle is perfectlybalanced about a horizontal axis so that it can swing in a plane of themagnetic meridian, the needle would make an angle with the horizontal(Fig. 5.10). This is known as the angle of dip (also known as inclination).Thus, dip is the angle that the total magnetic field B Eof the earth makeswith the surface of the earth. Figure 5.11 shows the magnetic meridianplane at a point P on the surface of the earth. The plane is a section throughthe earth. The total magnetic field at Pcan be resolved into a horizontalcomponent H Eand a verticalcomponent Z E. The angle that B Emakeswith H Eis the angle of dip, I.Magnetism andMatterFIGURE 5.10 The circle is asection through the earthcontaining the magneticmeridian. The angle between B Eand the horizontal componentH Eis the angle of dip.FIGURE 5.11 The earth’smagnetic field, B E, its horizontaland vertical components, H EandZ E. Also shown are thedeclination, D and theinclination or angle of dip, I.In most of the northern hemisphere, the north pole of the dip needletilts downwards. Likewise in most of the southern hemisphere, the southpole of the dip needle tilts downwards.To describe the magnetic field of the earth at a point on its surface, weneed to specify three quantities, viz., the declination D, the angle of dip orthe inclination I and the horizontal component of the earth’s field H E. Theseare known as the element of the earth’s magnetic field.Representing the verticle component by Z E, we haveZ E= B EsinI[5.10(a)]H E= B EcosI[5.10(b)]which gives,tan IZE=H[5.10(c)]E187