98 EXPLORATION GEOPHYSICS 



The inductor is usually mounted on a modified gimbal support so that 

 the axis of rotation of the coil may be placed in any direction in space. 

 The magnitude of the earth's total field, or any component of the total 

 field, may be computed from the magnitude of the current induced in the 

 coil and the constants of the instrument. The direction of the total field, 

 or one of its components, may be ascertained by noting the inclination of 

 the axis of rotation for zero current. 



A photograph of the Wild pattern earth inductor employed by the 

 United States Coast and Geodetic Survey is shown in Figure 29. The 

 instrument comprises essentially a coil of copper wire wound on a 

 cylindrical frame, a flexible shafting which is connected to the cylin- 

 drical frame, a commutator, a ring, and a graduated vertical circle 

 which is rigidly attached to the ring. The cylindrical frame is supported 

 by an axis (coil axis) that passes through a central diameter and rests in 

 bearings in the ring. The axis of the ring (inclination axis) is perpen- 

 dicular to the axis of the coil and is supported in a horizontal position on 

 bearings in uprights attached to the alidade. The graduated vertical circle 

 is parallel to the axis of the coil and concentric with the inclination axis. 



The azimuth of the coil axis may be adjusted by turning the ring about 

 the inclination axis, and the azimuth may be read by means of the 



Fig. 29. — Wild pattern earth inductor. A, vertical circle; 

 B, ring; C, coil; D, inclination axis; E, coil axis; F, brush 

 assembly and commutator; G, hand-cranked gear drive; H, slow 

 motion screw; I, binding posts; J, level; K, quarter sphere gears; 

 L, horizontal circle clamp. (Courtesy of the U. S. Coast and 

 Geodetic Survey.) 



graduated vertical circle and comparator or microscope shown in the right 

 hand portion of the figure. The coil may be rotated about its axis by means 

 of the flexible shafting, which connects one end of the axis with a gear, 

 and a hand crank. 



