MAGNETIC METHODS \77 



shaped sample of a certain thickness in comparison to a sample of great 

 thickness (theoretically of infinite thickness). A unifilar instrument was 

 employed, with a needle 10 cm. long and 3 mm. in diameter. The deflections 

 that should be obtained, on this magnetometer, for samples 1.2, 2.0, 3.0, and 

 6.0 cm. thickness were computed. The calculated deflections were compared 

 to those actually obtained from cut samples of these thicknesses, and the 

 results were found to agree within a few tenths of a scale division. 



It was also found that a block of material 6 cm. thick gave very closely 

 the same observed and computed deflection as one of (theoretically) infinite 

 depth. The dimension of 6 cm. was therefore taken as a standard with a 

 correction factor of 1. On this basis the sample 1.2 cm. thick had a factor 

 of 0.68. This meant that a specimen of that thickness would produce a 

 deflection of only 68 per cent of the deflection for the infinite case on which 

 the theory of the method is based. In like manner the 2.0 cm. thickness 

 and the 3 cm. thickness of test material gave correction factors of 0.85 and 

 0.93 respectively. 



On a similar basis, by computation and testing with shaped samples the 

 correction factor for the ratio of height and length was determined. From 

 the preceding, it is evident that the samples should be cut to uniform shapes 

 and sizes, preferably square and several inches on a side, in so far as 

 possible. 



These investigations also established the interesting fact that sometimes 

 there appears to be a magnetic anisotropy in certain types of rocks, mainly 

 crystalline schists. Such rocks show a maximum magnetic susceptibility 

 when the magnetic lines of force of the induced image of the magnetic 

 pole of the test instrument are parallel to the planes of rock schistocity. 

 Where the force lines from the induced pole are at right angles to these 

 planes the susceptibility is of a lesser amount. Gneissic granites also exhibit 

 this characteristic. 



Hy slop's Treatment of the Shape Factor. — In another technique, f 

 the shape factor becomes the basis for a field method of determining the 

 magnetic susceptibility of roughly shaped hand samples. In this method, the 

 deflection of a Schmidt vertical intensity field magnetometer is noted when 

 a test sample of rock is held close to one pole. Then by the application of 

 the dimensions of the sample (end area and height) to a set of curves which 

 relate dimensions and susceptibility, the latter may be estimated. 



Separate sheets cover sample heights or thicknesses of 2, 4, 6 and S cm. 

 Each graph sheet shows the variation in susceptibility for sample end sec- 

 tions of 2x2, 3x3, 4x4, 6x6, 7x7 and 8x8 cm. 



An example will illustrate the use of these sets of curves. A piece of 

 biotite granite was shaped roughly to a 3x3 cm. cross section and a height 

 of 4 cm. Readings of the field magnetometer were made with the test 

 block set over the pole in 8 diflferent orientations, showing an average 



t R. C. Hyslop, "A Field Method for Determining the Magnetic Susceptibility of Rocks," 

 A.l.M.E. Tech. Pub. 1285, Feb. 1941. 



