Dip Circle

The dip at a place can be measured using a dip circle. It consists of the following parts:

1.    The magnetic needle. It is mounted by means of a horizontal axis of hard steel that passes through the centre of gravity of the needle. The needle is free to move in a vertical plane. The axle about which the needle is pivoted is fixed to a vertical frame.

2.    The vertical circular scale. It is not front of the needle and the axis of the needle passes through the centre     of this scale. The scale is graduated so the both the ends of the needle can be read.

3.    Horizontal Circular Scale. The box containing the vertical circular scale and the magnetic needle can be rotated about a vertical axis and its position can be read on a horizontal circular scale. Three leveling screws are provided to make the horizontal circular scale perfectly horizontal. To find the dip at a place, the base board is first adjusted to be horizontal. To find the dip at a place, the base board is first adjusted to be horizontal with the help of the leveling screws and the spirit level. The box containing the circular vertical scale and the magnetic needle is rotated about the vertical axis until the needle points vertically up and down. In this position, the rotation of the needle is entirely due to the vertical component of the magnetic induction (Bv) due to the earth’s field. The horizontal component (BH) has no effect on the rotation of the needle. This shows that the vertical scale and the needle are in a plane at right angles to the magnetic meridian. Now the box is rotated through an angle of 90o with the help of the horizontal circular scale. This sets the needle exactly in the magnetic meridian. If the instrument and the needle are perfect, the reading of the magnetic needle gives the angle of dip at the place.