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:

Explanation

Let `B_V` and `B_H` be the vertical and horizontal components of Earth.s magnetic field respectively and `delta` be the true dip at the given place. Then:
`tan delta = (B_V)/(B_H)`
If the plane of dip circle is rotated and 8. is the apparent dip now, then
` tan delta. = (B._V)/(B._H)`
Vertical component `B._V = B_V`
Horizontal component `B._H = B_H cos theta`
`therefore tan delta. = (B_H tan delta)/(B_H cos theta) = (tan delta)/(cos theta)`
` rArr (tan delta.)/(tan delta) = (1)/(cos theta)`