Chap. 8] MAGNETIC METHOD 299 



moment oj the unit of volume and proportional to the generating field : 



^ = »cH (8-6) 



where the proportionality factor is the magnetic susceptibility. Substi- 

 tuting (8-6) in (8-5) and dividing by H, y = 1 + ^ttk so that 



K=^- 



1 



4ir 



(8-7) 



A body has the unit of susceptibility when its unit of volume receives the 

 moment 1 by the field 1. The susceptibility of vacuum is zero. Para- 

 magnetic substances have positive susceptibilities, diamagnetic substances, 

 negative. Since only the strongest magnetic minerals approach the unit 

 of susceptibility, it is customary to employ 1 millionth part of this unit in 

 practical work. 



Magnetic susceptibility and permeability are not constants but depend 

 on the strength of the magnetic field. It is customary to plot the induc- 

 tion B or the intensity of magnetization d as function of field strength. 

 Flux density is generally measured in maxwells per cm^ or kilo-lines per 

 square inch (1 kilo-line per square inch = 155 maxwells per square cm, 

 or gauss) , It is also obtained from the intensity of magnetization by multi- 

 plication with 4x. Field intensity is measured in gauss or ampere turns 

 per inch (1 ampere turn per inch = 0.495 gauss). 



A graph showing the magnetization as function of (positive and nega- 

 tive) fields is known as hysteresis curve (Fig. 8-2). OA is the virgin portion 

 obtained after a specimen has been demagnetized. When the field strength 

 is decreased (after the specimen is magnetized to saturation), this curve 

 is not retraced. Hence when the field strength is zero, the induction is not 

 zero and retains the value OB, whicli is the remanent or residual magnetiza- 

 tion. The negative field required to reduce the magnetization to zero is 

 the coercive force, OC. When the field is further increased in the negative 

 direction, the reflected curve is obtained. 



B. Methods of Determining Rock Magnetization 



Rock specimens are usually given a preliminary test to determine the 

 proportions of residual and induced magnetization. Next, their suscepti- 

 bility may be measured' at field strengths equal to those of the earth's 

 field components. Further tests involve measurements of susceptibilities 

 at higher field strengths, that is, investigations of complete hysteresis 

 curves. 



1. For a preliminary determination of the induced and remanent mag- 

 netization, the rock sample is placed close to a magnetometer and the 

 deflection at distance r is noted. Then the sample is rotated through 180 



