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A dip circle lies initially in the magne...

A dip circle lies initially in the magnetic meridian. If it is now rotated through angle `theta` in the horizontal plane, then tangent of the angle of dip is changed in the ratio:

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As `tan phi = (B_(v) /B_(H))` so when the needle is vertical, i.e., `phi = 90^@`, `B_(H) =0`.i.e., the plane of dip circle is perpendicular to the magnetic meridian. Now if the plane of dip circle is rotated by `alpha` from this position, the angle between magnetic meridian and plane of dip circle will be `(90^@- alpha)` and as angle of dip in this plane is `delta`,
`tan delta=B’_(v)/B’_(H)=B_(v)/B_(H)cos(90 - alpha)` or `tan delta=tan phi/sin alpha i.e..,phi=tan^(-1)(sin alpha tan delta)`
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