MAGNETIC METHODS 175 



Field Magnetometer Method. — A direct comparison of similar vol- 

 umes and shapes of material of known and unknown magnetic susceptibil- 

 ity can be made by using a field magnetometer. The procedure is essen- 

 tially the same as that descril)ed under the test tube method. 



For such measurements the magnetometer to be used is adjusted so that 

 it has a high sensitivity. (This adjustment was given in detail in a pre- 

 ceding section.) An instrument of high sensitivity is necessary in order 

 that a measurable response can be obtained from test samples and their 

 equivalent volumes of standard susceptibility material. Where the con- 

 struction of the particular magnetometer permits, its outer casing should 

 be removed so that the sample can be brought as close to the magnetic 

 system of the instrument as possible. 



In one set of such tests an old-style Askania vertical intensity field mag- 

 netometer was used. It was adjusted so that one scale division movement 

 of its magnetic system represented a change in magnetic field of 11.4 x 10~^ 

 gauss. Such a sensitivity is about three times that usual for an instru- 

 ment used in oil exploration field work. Before starting observations on 

 the samples the instrument was carefully set up and its outer housing 

 removed. 



The first rock on which measurements were made was an oblong piece 

 of amphibolite whose dimensions were 2.2 x 1.6 x 2.2 inches. It had smooth 

 faces as it had been cut from a larger sample with a rock saw. This test 

 sample was carefully placed on top of the magnetometer above the north 

 pole of its magnetic system. In this position the face of the sample in con- 

 tact with the instrument was 1.2 inches from the magnetic pole. A series 

 of readings of the instrument scale was made, (1) with the sample in place 

 and (2) with the sample removed, to determine the efifect produced. Read- 

 ings were taken with dififerent faces of the sample in contact with the top 

 of the instrument. It was found that the block of amphibolite caused an 

 average change in scale reading of 12.0 scale divisions. 



For the second part of the experiment an equal volume of iron chloride 

 solution was put in a non-magnetic (copper) vessel of the same shape and 

 size as the sample, and set in exactly the same place, on top of the magneto- 

 meter. With the box (1) in place and (2) removed, a number of readings 

 were made which showed that the iron chloride gave an average scale 

 deflection of 0.7 scale divisions. 



To obtain a volume of standard susceptibility solution in an exactly 

 similar form to the rock sample the following procedure was used. The 

 cut sample of rock was put into the corner of the copper vessel and the 

 remaining space in the container was filled with moulding clay. The rock 

 was then removed and the space it had occupied was filled with the iron 

 chloride solution. 



The solution of iron chloride used in these tests was of 46.45%, and it 

 had a volume susceptibility of 60.42 x 10"*^ c.g.s., as calculated previously. 



The magnetic susceptibility of the sample of amphibolite is related to 



