MAGNETIC METHODS 



173 



when subjected to an alternating magnetic field, and would be suitable for 

 transformer cores, (c.) The flatness of the curves and the large area is 

 indicative of steel of high retentivity and coercive force which could be used 

 in magnetic systems. 



The coercive force of steel magnets increases with the carbon content. 

 Tungsten, nickel and molybdenum also increase the coercive force in steel. 

 Manganese reduces the magnetic properties of iron, and with 15.2% of 

 Mn present the alloy is non-magnetic. 



(b) LAMINATED TRANSFORMER 

 IRON 



(c) STEEL OR ALLOY STEEL 

 PERMANENT MAGNET 



(q) SOFT IRON 



Fig. 73. — Hysteresis curves for three different types of magnets, 



Measurement of Rock Magnetization 



A number of methods have been developed for determining quanti- 

 tatively the magnetic susceptibility of rocks. These are mainly laboratory 

 procedures that require special apparatus. There are certain techniques, 

 however, that can be used in the field, such as the one worked out by R. G. 

 Patersonf for determining the susceptibility of rocks in place. These 

 latter methods are particularly useful. A knowledge of the magnetic char- 

 acter of the dififerent types of rock that may be involved in a given field 

 problem is often of much importance in interpretation. 



Test Tube Method. — One procedure for finding the relative susceptibility of a 

 rock sample is called the test tube method. The instrument used comprises essentially 

 a simple unifilar magnetometer consisting of a slender magnetic needle, about 10 inches 

 long, which is attached at its center to a fine torsion fiber of about the same length. The 

 less the stiffness of the torsion fiber, the greater the sensitivity. A suitable housing 

 protects the fiber and needle from air currents, and a torsion head, to which the suspen- 

 sion fiber is attached at its upper end, permits the needle to be oriented. The instrument 

 is equipped with a telescope and scale so that the deflections of the suspended system, to 

 which a small mirror is fastened, can be accurately observed. In this respect it is 

 similar to the familiar wall-type d'Arsonval laboratory galvanometer. 



A base plate with leveling screws and sinall feet or a light tripod makes it possible 

 to use this kind of test equipment in the field, although it is primarily a laboratory 

 instrument. The essential features of the apparatus described above are shown in 

 Figure 26. 



With such a magnetometer, a susceptibility measurement is made by first placing 

 a measured volume of pulverized sample of the rock under investigation in a test tube. 

 The tube is then brought to within a few centimeters of one pole of the magnetic needle 



t R. G. Paterson, "Determination of Magnetic Susceptibilities of Rocks in Situ," 

 Tech. Pub. No. 1298, Feb., 1941. 



A.I.M.E. 



