180 EXPLORATION GEOPHYSICS 



residual magnetization. The amount of such magnetization could be found 

 by measuring the current in the solenoid needed to reduce the deflection to 

 zero. 



The magnetic moment of test samples of known volume can be deter- 

 mined quantitatively at given magnetizing fields. The latter are set up by a 

 measured current in the primary windings of the two solenoids. The 

 amount and direction of these fields can be controlled, permitting the hyster- 

 esis loop for a given test piece to be run. As indicated, the moment of a 

 sample when in the solenoid is obtained in terms of the field, in the 

 secondary winding, required to balance the magnetic system of the instru- 

 ment to zero deflection. 



From formulae previously given: M = I V ; or magnetic moment of a 

 sample M is its intensity of magnetization / times its volume V. The 

 neutralizing or balancing field (in the secondary of the solenoid) is H. 

 H — 2Md/((P ~ l^y. In this equation d equals the distance from the 

 center of the sample to the magnet of the instrument, or system. The 

 length of the test sample is 21. The value of H is found from the milli- 

 amperes of current in the secondary of the solenoid carrying the sample. 

 The dimensions of the solenoid in length and turns of wire per centimeter 

 are known. 



Tests were run by Loring on a number of rocks of different types and 

 on other materials. It was found, for example, in the case of a sample 

 of granite, 2" x 2.5'^ x S" , that there was no residual magnetization present. 

 The magnetic susceptibility of this granite was constant at a value of 474 

 x 10~^ c.g.s. for fields up to 12 gauss. This would signify that the hysteresis 

 loop had no area, but was represented by a straight line across the B-H 

 graph at a constant slope ; and such was the case when the data was plotted. 



A sample of 50 per cent iron chloride solution, in a copper box, 2" x 2'' 

 x V when tested in the instrument gave similar results. The hysteresis 

 curve was a straight line of constant slope. This finding indicates the ad- 

 vantage of using iron chloride for standardization and comparison in these 

 tests, as its susceptibility does not change in magnetic fields of different 

 strength. 



Two samples of basalt, one (1''' x 2" x 3") from an intrusive dike and 

 the other (1.5'^ x 2" x 2.5") from a flow, gave susceptibilities of 1420 x 

 10~^ and 1115 X 10~^ c.g.s. respectively. They both showed remanent mag- 

 netization of 0.01 gauss which was removed by a coercive force of 1.3 

 gauss. 



A block of moulding clay 2" x 2" x 6" in size, with which iron filings 

 had been thoroughly mixed, exhibited a hysteresis loop that enclosed a 

 sizable area and otherwise showed magnetic properties similar to a natural 

 rock. This specimen had an initial remanent magnetization, before tests 

 were run, of 2 gauss. The coercive force to obtain zero magnetization was 

 2.26 gauss. Some variation in susceptibility with magnetizing field was 

 evident, but for a field of 0.9 gauss the susceptibility was 3500 x 10~®, 



