A new method of measuring the inclination of the earth's magnetic field

Abstract

The instrument is based on the increase in impedance, due to the skin effect, caused by a decrease in axial component of magnetic field in a wire of high permeability nickel iron when carrying alternating current. Four straight wires of similar characteristics are arranged accurately in a square, and form the four arms of an A.C. bridge. If the plane of the assembly lies in the magnetic meridian plane and is then rotated about a horizontal axis it is evident that when the total field vector lies in one of the diagonals of the square the bridge automatically balances and the angle between the diagonal and the horizontal plane is the angle of dip. Such an all-magnetic bridge needs no ancillary balancing adjusters. In the instrument the four wires are mounted as a square assembly in a disc, which is itself mounted in a ring within which it can rotate about an axis lying in one of the diagonals of the square. The ring is supported in a base block in such a manner that by means of a worm and micrometer head the whole ring (carrying the disc) can be rotated about an axis through its centre, perpendicular to the plane of the ring. The instrument, owing to its inherent geometrical simplicity, eliminates errors due to: - want of perfect match of the wires; inaccuracy of geometric mounting of the wires; the vertical axis not being normal to the base plane. The value of the dip is independent of the adjustment of the electrical unit provided this does not occur between the times of the direct and reversed readings necessary to a determination of the dip.