MAGNETIC METHODS 63 



ber of others, notably, ilmenite, pyrrhotite and chromite. Artificial magnets 

 are bodies which can be made into magnets by bringing them into a strong 

 magnetic field, or by stroking them with other magnets. Examples of such 

 substances are steel and iron, and to a lesser degree nickel, and cobalt. Cer- 

 tain alloys of non-magnetic substances can acquire magnetism. For exam- 

 ple, one such alloy consists of 26.5% manganese, 14.6% aluminum and 

 58.9% copper. 



On the basis of their ability to retain their acc[uired magnetism, artificial 

 magnets are likewise divided into two groups, namely ; permanent and 

 temporary. Permanent magnets are generally made of hard steel or steel 

 alloy, and possess high retentivity or ability to keep their magnetism. Steels 

 of this kind are selected for compass needles and the permanent magnetic 

 parts of the systems of magnetic instruments. Temporary magnets are 

 made of substances such as soft iron and certain alloys of iron which 

 become magnetic when placed in a magnetic field, but which lose their 

 magnetism upon removal from that field. Temporary magnets are said to 

 have little retentivity. 



Classification of Substances. — All substances may be classified mag- 

 netically according to their behavior in a magnetic field as diamagnetic, or 

 paramagnetic. The basis for such a classification was established in 1845 

 by Faraday, who demonstrated that all materials are affected to some extent 

 by a strong magnetic field. 



Certain paramagnetic substances that have very pronounced magnetic 

 properties are put in a special group known as ferromagnetic. These latter 

 are of particular importance geophysically. 



Diamagnetic materials show a mild response to a magnetic field and 

 the efifect is said to be repellent. Substances can be tested, in a simple 

 manner, by suspending them in a magnetic field of suitable strength and 

 form. This is illustrated in Figure 5 (after Loeb)f which shows that in a 

 divergent magnetic field diamagnetic materials tend to move from the 

 stronger to the weaker parts of the field. They are repelled or pushed away 

 from the field. 



When pieces of diamagnetic material, of oblong form, are placed in 

 a uniform field, they try to set their long dimension at right angles to the 

 lines of force of the field. As will be described in detail later, the mag- 

 netic force lines of the field diverge from such materials. 



Typical of the diamagnetic materials are bismuth, copper, rock salt, 

 anhydrite, etc. 



Paramagnetic materials show a definite attraction to a magnetic field, 

 and the efifect is described as attractive. Under test conditions (Figure 5), 

 paramagnetic substances move from the zveaker to the stronger portions of 

 a divergent field. Oblong pieces of this kind of material tend to place 

 their long dimension parallel to the lines of force of a uniform magnetic 



t L. B. Loeb, Fundamentals of Electricity and Magnetism, John Wiley, 1931, p. 224. 



