William Watson

“ Dip Circles.—In the case of the dip circle the direction of the earth’s magnetic field is obtained by observing the position of the axis of a magnetized needle so supported as to be free to turn about a horizontal axis passing through its centre of gravity. The needles now used consist of flat lozenge-shaped pieces of steel about 9 cm. long and 0.1 cm. thick, and weigh about 4.1 grams. The axle, which is made of hard steel, projects on either side of the needle and has a diameter of about 0.05 cm. ”

“ Hence if the axis about which such a coil spins is adjusted till a sensitive galvanometer connected to the coil through a commutator, by which the alternating current is converted into a direct current, is undeflected, then the axis must be parallel to the lines of force of the earth’s field, and hence the inclination of the axis to the horizontal is the dip. ”

“ There are also two adjustable stops which can be set in any position, and allow the upper part of the instrument to be rotated through exactly 180° without the necessity of reading the horizontal circle. ”