176 EXPLORATION GEOPHYSICS 



the above experimental data in a direct proportion: the deflection due to 

 the rock sample is to the deflection caused by a like volume of iron chloride 

 as the susceptibility of the sample is to the susceptibility of the iron chloride 

 solution. In equation form : 



12.0 : 0.7 = X : 60.42 x 10-^ 

 To solve : 



12/0.7 X 60.42 X lO"*' = 1033 x lO-^ c.g.s. 



The volume susceptibility of the particular sample of amphibolite rock 

 is therefore 1033 x 10~*' or 0.001033 c.g.s. Such a value would indicate a 

 highly magnetic material. Another amphibolite, which is described as fairly 

 rich in magnetite, had a value of 1130 x 10"^. 



Among the rocks tested by this method, a specimen measuring 2.3 x 

 1.7 X 1.8 inches on a side, and from the same general area which was called 

 paleozoic rock, was found to cause no deflection on the magnetometer. Its 

 equivalent shape and volume of iron chloride solution gave a deflection of 

 0.3 scale divisions. From this somewhat negative result, it can be concluded 

 that the paleozoic rock had a susceptibility less than the 60.42 x lO^*' of the 

 solution. It follows, however, from these two isolated examples that in 

 the general area where such paleozoics and amphibolites were involved any 

 magnetic anomalies present would, in all probability, be associated with and 

 arise from the amphibolites. 



The Shape Factor in Test Samples.— The background for suscep- 

 tibility measurements where a plane surface of a cut sample is placed near 

 the pole of the system of a magnetometer has been given by J. G. Koenigs- 

 berger.f He applies the principle of the theory of electrical images to the 

 magnetic case here involved. 



The solution of the problem brings out the following facts : that the 

 change in the magnetic field due to the presence of the test sample (as ex- 

 pressed in terms of instrument scale deflection) is (1) directly proportional 

 to the susceptibility of the sample; (2) directly proportional to the pole 

 strength of the system in the magnetometer; (3) inversely proportional to 

 the square of the distance from the magnetic pole to the near face of the 

 test specimen ; and (4) dependent upon the shape factor and size of the 

 specimen. In equation form : AD = -KinkS/lr^, where AD is the change in 

 magnetic field or the deflection of the instrument caused by the sample; m 

 is the pole strength of the magnetic system of the magnetometer used ; k is 

 the susceptibility of the test sample, r the distance from the magnetic pole 

 to the face of the sample, and 5 the shape-size factor. 



The main point developed by Koenigsberger is that the above relation 

 is true only for an infinite body and that for shaped samples of a relatively 

 small thickness corrections relating to their dimensions are necessary. 

 Experiments were conducted to determine the proper correction for a 



t J. G. Koenigsberger, "Method for Measuring the Susceptibility of Rocks," Terr. Mag. &■ 

 Atmos. Elec, Vol. 34, p. 210, Sept. 1929. 



