174 EXPLORATION GEOPHYSICS 



and held at an accurately measured distance. The deflection caused by the test material 

 is observed. This material is then removed and a like volume of some material of 

 known magnetic susceptibility (preferably in the same test tube) is placed at the same 

 position in respect to the needle. The deflection caused by the substance of known 

 susceptibility is then observed. The comparison is a direct one. The deflections caused 

 by exactly similar volumes of material, at exactly the same position and distance from 

 the needle of the instrument, are directly proportional to their magnetic susceptibilities. 



This can be seen as follows : I = k H ; therefore, for a given H (e.g., 

 the earth's field) the larger the k, the larger the /. But B = H + Airl, and 

 thus, the larger the k, the larger the B field, and the greater the field acting 

 on the magnetometer needle. An example of such a determination follows 

 a discussion of the standards of susceptibility with which test samples can 

 be compared. 



The horizontal component of the earth's magnetic field is the force which acts to 

 orient the needle of a unifilar magnetometer of the form described when no magnetic 

 samples are near by. It is, in effect, one type of compass. The sensitivity of such an 

 instrument can be much increased if a bar magnet, of a strength sufficient to produce a 

 field somewhat less than that of the earth's horizontal component, is placed below the 

 magnetometer. The bar magnet is set in a horizontal position parallel to the needle and 

 so oriented that its field opposes, and thus tends to compensate, the said horizontal 

 component of the earth's field. In this way a smaller resultant or controlling field is 

 acting on the needle. When a torsion fibre of small torsional coefficient is used, the 

 system becomes responsive to the very small changes in the field created when samples 

 of materials of different susceptibility are positioned near the needle. 



Standards of Susceptibility, — In determining the magnetic suscept- 

 ibility of test samples, standards of known value are needed. One of the 

 most useful materials for this purpose is ferric chloride. 



The mass susceptibility of a ferric chloride solution of known per cent may be 

 calculated from the following formulaf. 



k (mass) = (p/100) H + (1- p/100) Ho (66) 



wherein H = 90 x 10"*' is the mass susceptibility of pure ferric chloride, Ho = the mass 

 susceptibility of water or — 0.79 x 10"^, and p = percent iron chloride. 



By this formula a solution of 46.45 percent iron chloride had a mass susceptibility 

 of 41.38 X 10'^ c.g.s. Its density was 1.46, so that the volume susceptibility for this 

 solution was 60.42 x 10"^ or 0.00006042 c.g.s. units. This figure was obtained by multi- 

 plying the mass susceptibility by the density. Iron chloride solutions of percentages of 

 the order of the example cited make suitable standards for comparison measurements 

 on rocks and minerals. 



Such solutions should be carefully prepared and stored in a tightly-stoppered dark 

 glass bottle for protection against changes caused by light and oxidization. A solution 

 of such a high per cent (approaching 50%) of iron chloride is not entirely stable; some 

 of the iron chloride may change to the ferrous form, altering the initial percentage 

 value, and hence the susceptibility. A solution somewhat less than 50 per cent ferric 

 chloride is about as high a concentration as will remain in a stable condition. It is desir- 

 able that the percentage and density of the solution be determined from time to time 

 to guard against changes. 



t The Smithsonian Physical Tables, 8th Edition, 1933, p. 475. 



