Chap. 8] MAGNETIC METHOD 403 



Haanel'^ describes an arrangement for magnetic model experiments con- 

 sisting of a square table about 40 inches high and 65 inches square, brass 

 rails on two sides, on which a bridge could be moved to any desired posi- 

 tion. The bridge carried a movable slide to which a magnetometer was 

 attached. Rails and bridge were graduated so that the position of the 

 magnetometer with reference to any stationary object below the table 

 could be accurately determined. The model ore bodies were made of hard 

 cast iron or hardened steel, and they could be magnetized by solenoids. 

 Their distance from the plane of observation was regulated by supports of 

 properly adjusted heights. A vertical adjustment of the position of the 

 magnetometer within small limits made it possible to investigate the effects 

 of topographic irregularities. 



Hotchkiss^^ studied the influence of dip and strike of magnetic forma- 

 tions upon a dial compass and a dip needle on a simple drawing board. 

 On the underside of this board a sheet of tin 20 by 28 inches was attached 

 by suspending it on a pair of copper nails in varying distances and at 

 varying angles of dip. For the determination of variations in declination, 

 a compass was fastened to a triangle, and this triangle was moved along a 

 T square. With a more elaborate experiment table adapted to observa- 

 tions with both dial compass and dip needle, he then made measurements 

 of the influence of dip and strike upon both declination and dip. 



Rothelius,^' in 1924 used an elliptical magnet 35 cm long, 7 cm wide 

 at the largest point, and 8.5 mm thick. Eve and Keys^°° experimented 

 with vertical sheets to check methods of determining depths and dimen- 

 sions of magnetic ore bodies from observations in different levels. ^"^ 



An efficient arrangement for testing magnetic structures, effects of base- 

 ment rocks, and the like, has been perfected by Barret. ^°^ The models 

 are made of a plastic substance of variable magnetic susceptibility and are 

 placed in a tray at an adjustable distance from a glass plate representing 

 the earth's surface. The earth's magnetic field is produced by a pair of 

 Helmholtz coils which may be tilted and may be supplied with variable 

 alternating currents to simulate changes in the direction and intensity of 

 the normal earth's magnetic field. The normal field is first measured 

 without the model by a small earth inductor connected to a rectifier and 

 D.C. galvanometer. Then the model is inserted and the measurements 

 are repeated. Since the distance between the glass plate and the model 



" Op. cit., 95. 



'* Wisconsin Geol. and Nat. Hist. Survey Bull., 44, Chap. IV, "Magnetic Obser- 

 vations," pp. 112-113 (Madison, 1915). 



89A.I.M.E. Geophysical Prospecting, 210-211 (1932). 



lo" A. S. Eve and D. A. Keys, A.I.M.E. Geophysical Prospecting, 206 (1932). 



101 See also Canad. Geol. Survey Mem. 170, 30 ff. (1932). 



"2 Oil Weekly, 66(7), 27-30 (Aug. 1, 1932). 



