178 EXPLORATION GEOPHYSICS 



deflection of 13.08 scale divisions. The sensitivity of the instrument was 

 15 X 10~^ c.g.s. units per scale division, which gave a figure of 196.3 

 gammas for the scale reading (15.0 x 13.08 = 196.3). 



The graph for the 4 cm. height of sample was selected. On this graph 

 the ordinate at 196.3 was traced upward to its intersection with the curve for 

 the 3x3 cm. cross section and then across to the susceptibility axis. The 

 value of susceptibility of 4200 x 10~^ was obtained. 



The problem of finding a material of known susceptibility that could 

 be conveniently cut into blocks of desired end section dimensions and height 

 was solved by the use of mixtures of iron filings and moulding clay 

 (plastocene). Five batches of clay with dififerent percentages of iron filings 

 thoroughly mixed gave a set of permanent susceptibility standards covering 

 a wide range of values. The susceptibility of the clay-iron filings mixtures 

 was determined by direct comparison with an iron chloride solution of 

 known per cent. 



A great number of instrument readings (about 2400) were required 

 to build up the sets of curves of susceptibility vs. scale reading for the four 

 thicknesses (heights) and the seven cross sections used. 



Amhronn's Two Solenoid Method. — Several investigatorsf have 

 devised various laboratory methods and equipment for determining rock 

 susceptibilities by the use of solenoids. One operating principle may be 

 described as follows. 



Two identical solenoids are mounted symmetrically on either side oi 

 the lower magnet of an astatic magnetic system (Figure 74). An astatic 

 system as here devised has two short magnets attached by a light rod. The 

 system is suspended on a delicate torsion fibre. The magnets are placed 

 with their poles reversed in respect to each other. Theoretically, and prac- 

 tically to a high degree, any changes in the horizontal magnetic field (earth's 

 field) acting on the system would be neutralized and would not twist the 

 suspended system of the instrument. This is because such a change would 

 tend to turn the upper magnet in one direction and the lower magnet in the 

 opposite direction, with the two torques acting upon each other through 

 the rod attaching the two magnets. The solenoids are series-connected, near 

 poles in opposition, and the system is carefully balanced by adjustment of 

 the potentiometer until the magnetometer deflection is independent of the 

 current flowing through the solenoids. 



The test specimen 5" is now inserted in one solenoid. The specimen may 

 be ground to size or pulverized and placed in a glass specimen tube. When 

 current is passed through the coils, the dififerent permeability of the 

 specimen (compared to the air which previously filled the solenoid) upsets 

 the magnetic balance of the solenoids and produces a deflection of the astatic 

 magnetometer system. This magnetometer deflection is reduced to zero 



t R. Ambronn, Elements of Geophysics, (Translated from the German by Margret Cobb), 

 McGraw-Hill, 1928, p. 92. 



