MAGNETIC METHODS 



79 



a magnet a given magnetizing field will produce in a substance in relation 

 to the strength of the magnetizing field applied. 



It is apparent that these two magnetic properties relate to somewhat 

 different aspects of the magnetic character of materials. 



These matters have specific application in magnetic exploration. The 

 magnetic strength and hence the detectability of a magnetic body, such as 

 a magnetic dike, for instance, depend on its susceptibility and the contrast 

 in susceptibility between it and the surrounding rock. See Figure 13. 



Fig. 13. — Distortion of earth's field due to various materials: 

 (a) non-magnetic material; (b) paramagnetic or ferromagnetic 

 material having a susceptibility greater than the surrounding 

 medium; (c) diamagnetic material, with a susceptibility less than 

 the surrounding medium. 



Magnetic Intensity Due to a Magnet 



The operation of many magnetic instruments, and the calibration of 

 Schmidt type magnetometers with auxiliary magnets, require the quanti- 

 tative determination of the magnetic intensity at a measured distance from 

 a simple bar magnet. There are three cases or positions of Gauss that 

 adequately cover the usual situations. These are as follows : 



Magnetic Force at a Point Due to a Bar Magnet: 



Case I. Magnet end on, or Position I of Gauss. (See Figure 14.) 



• p 



Fig. 14. — Magnetic force at a point due to a bar magnet: Position I of Gauss. 



Hn 



m m _ m(d^+ 2ld + P) - m {d- - 2ld + P) 



(d-l)' (d + iy~ (r/2-/2)2 



4 m Id 2M d 



^^ (^2_;2)2 



(d^-py 



(20) 



