MAGNETIC METHODS 



207 



the board and at different angles of dip. The magnetic declination was 

 measured with a compass at various positions along "traverses" parallel 

 to the sides of a wood T square. The sheet could also be oriented at dif- 

 ferent strike angles with the magnetic field. 



The Keys' experiment was a laboratory investigation of the magnetic 

 effects of a "thin magnetized dike of limited length." t A. S. Eve, t in 

 collaboration with Keys and several other investigators, had carried out 

 a survey over a portion of the Falconbridge ore body in which two sets 

 of measurements were made with an Askania magnetometer, one set being 

 made on the ground and the other on elevated platforms. The purpose of 

 the survey was to determine the lower depth of the ore body from the 

 differences in the anomalies at the two levels (ground and platform). The 

 survey did not yield an accurate value of the lower depth, and the Keys' 

 experiment was designed to verify the correctness of the method of inter- 

 pretation used in the survey. 



60 



55 



50 



-^45 



r40 



I 35 



E 



I 25 

 o 



t;20 



C 



115 

 10 

 5 

 



Distance from Center of Dike, centimeters N 



(a.) 



Lh) 



Fig. 98. 



(a) Sketch of narrow magnetized dike of limited dimensions. 



(b) Experimental and tlieoretical vertical magnetic anomalies at two different levels over a model 

 magnetic dike. (Keys, A.I.M.E. Geophysical Prospecting, 1932, p. 206.) 



A sheet of mild steel, which was approximately one-half inch thick, 

 four feet high, and ten feet long, was supported vertically on its edge with 

 its length perpendicular to the magnetic meridian. To magnetize the sheet 

 ten turns of insulated copper wire were wound lengthwise around its cen- 

 ter and a current of one ampere passed through the wire in a direction to 

 make the upper edge a south pole. An Askania vertical variometer was 



t D. A. Keys, loc. cit. 



t A. S. Eve, "A Magnetic Method of Estimating the Height of Some Buried Magnetic Bodies, 

 A.I.M.E. Geophysical Prospecting, 1932, pp. 200-205. 



