100 EXPLORATION GEOPHYSICS 



rotated. In this position, the axis of the coil is parallel to the resultant 

 or total field of the earth, and the angle of inclination of the coil axis as 

 measured on the vertical circle is the dip. 



If only the direction of the earth's field is to be measured, any type 

 of sensitive null-point galvanometer (string, loop, astatic, etc.) may be 

 employed. If the strength or intensity of the field is to be determined, a 

 ballistic galvanometer is usually employed. (If intensity measurements are 

 based on the potential compensation principle, however, null-point gal- 

 vanometers are again usable.) 



Another type of portable earth indicator and galvanometer has been 

 designed and constructed by the Carnegie Institution of Washington. The 

 galvanometer is so designed that the arm carrying the telescope and scale 

 can be folded and carried in the earth-inductor carrying case, t Peep 

 sights and a compass attachment are sometimes provided for setting the 

 earth inductor in the magnetic meridian. If the magnetic declination and 

 the azimuth of a reference mark are known, the meridian setting may be 

 made with the peep sights alone. Ordinarily, however, it is necessary to 

 use the compass attachment. Various instruments have been developed for 

 determining the magnetic declination. Generally, the magnetic declination 

 is measured by two operations : ( 1 ) determining the true meridian by 

 latitude observations at noon, if the longitude is known approximately; and 

 (2) determining the magnetic meridian. One instrument for determining 

 the magnetic meridian is illustrated in Figure 30, and is often referred to as 

 a theodolite magnetometer. This instrument comprises a long hollow 

 magnet suspended by a torsion fibre. A telescope and scale measure the 

 angular deflection of the magnet system. 



Special Devices for Determining the Magnetic Gradient, 

 Intensity, and Meridian 



Magnetic Gradiometer. — The magnetic gradient is measured by com- 

 paring the induced electromotive forces in two similar coils spaced apart 

 with parallel axes of rotation and rotatable at the same speed.t If the 

 magnetic field intensities perpendicular to the axes of rotation are 

 different at the two coils, the relative magnitudes of the electromotive 

 forces induced in the coils will depend on the relative magnitudes of the 

 field intensities. The two coils of the gradiometer are mounted in a frame 

 and are free to rotate about axes at right angles to the length of the 

 frame. The ratio of the E.M.F.'s induced in the two coils is measured 

 by connecting the coils to opposite arms of a Wheatstone bridge circuit. 

 A sensitive galvanometer is employed for indicating the null-point.* 



t Hazard, loc. cit., pp. 80-93. 



j J. Roman and T. C. Sermon, "A Magnetic Gradiometer," A.I.M.E. Geophysical Prospecting, 

 1934, pp. 373-388. 



* For a derivation of the magnetic gradients as a function of the ratio of the 

 induced E.M.F.'s, see Roman and Sermon, loc. cit., pp. 381-384. 



