Chap. 8] MAGNETIC METHOD 363 



of earth inductor observations with record forms are given by 

 Hazard. 



The use of the earth inductor as a compass is based on its ability to find 

 the magnetic meridian by the zero-induction brush position (see page 361). 

 In application (airplane) the brushes are so set as to make the required 

 course angle with the axis of the ship; the pilot then navigates the ship 

 in such a manner that the galvanometer on the instrument board shows no 

 deflection. 



Magnetic intensities may be measured with the- earth inductor by using 

 the ballistic method previously discussed, by keeping the speed of the coil 

 constant, or by compensating the field so that the speed is no longer of 

 influence. In the constant-speed method the coil is coupled to a syn- 

 chronous motor driven from a valve-maintained tuning fork, and the e.m.f. 

 induced in the coil is measured with a potentiometer. The compensation 

 principle is applied in an inductor provided with a pair of Helmholtz 

 coils mounted to the frame B in such a manner that the field of the coils 

 opposes the earth magnetic field in the z" direction. The compensation 

 current is accurately measured by a potentiometer arrangement. A dis- 

 advantage is the lack of homogeneity in the inside of the coil. This lack 

 may be overcome by the new form of Helmholtz coils suggested by Fan- 

 selau and Bock.^'' In a compensation inductor suggested by the author,^^ 

 a large magnet compensates the major portion of the field and a smaller 

 magnet is moved along a deflection bar for compensation. This instru- 

 ment is suitable for any intensity component. The possibilities of the 

 compensation inductor are well demonstrated by the fact that it has 

 been proposed as a primary standard in absolute magnetic observatory 

 work.''" 



The earth inductor gradiometer is intended for the determination of 

 "intensity gradients" in the north and east directions. It consists of a 

 horizontal frame in which two coils rotate about horizontal axes. They 

 are situated on opposite arms of a KirchhofT-Wheatstone bridge so that the 

 ratio of the e.m.f. 's induced in the coils is observed. The Null instrument 

 is a sensitive galvanometer set up on a separate tripod. When the coil 

 frame is E-W, the axes of the two coils are N-S, which eliminates the 



*^ Op. cit., pp. 77-84. 



^8 F. M. Soule, Terr. Mag., 35(2), 103-110 (June, 1930). 



*9W. Uljanin, Terr. Mag., 34(3), 199-206 (Sept., 1929). 



"See G. Fanselau, Zeit. Phys., 54(3/4), 260-269 (1929). F. E. Smith, Phil. 

 Trans., A223, 186-191 (1923). R. Bock, Zeit. Phys., 54, 257-259 (1929). R. H. 

 Bacon, Rev. Sci. Instr., 7(H), 423 (Nov., 1936). 



" See P. Aguerrevere, Colo. School of Mines Quarterly, 27(3), 11-29 (July, 1932). 



"« E. A. Johnson, Terr. Mag., 44(1), 29-42 (1939). 



" See I. Roman and T. C. Sermon, "A Magnetic Gradiometer," A.I.M.E. Geo- 

 physical Prospecting, 373-390 (1934). 



