MAGNETIC METHODS 65 



relatively more magnetic character of their surroundings. Diabase is ordin- 

 arily magnetic due to its magnetite content. 



Some writers set up a slightly different magnetic classification of mater- 

 ials. In it the term non-magnetic is essentially synonymous with diamag- 

 netic, as here used, and magnetic refers to both paramagnetic and ferro- 

 magnetic substances. Such terminology relates to the divergence of mag- 

 netic lines of force away from diamagnetic bodies* and their convergence 

 into paramagnetic bodies. Certain materials are non-magnetic in the sense 

 that lines of force do not easily penetrate them, and they cannot be made 

 into magnets. A magnetic substance, however, attracts lines of magnetic 

 force and in other respects acts like and may become a magnet. The system 

 corresponds to popular usage and thought, for we ordinarily consider steel 

 or iron as magnetic and wood and glass as non-magnetic. 



The classification of magnets and magnetic substances or materials 

 here presented can be summarized in the following table. 



Magnetic Classification 



Magnets Bodies that attract iron 



a. Natural magnets Magnetite 



b. Artificial magnets Magnetized steel 



(1) Permanent magnets Compass needle 



(2) Temporary magnets Soft iron 



Substances** 



a. Diamagnetic Glass, rock salt 



b. Paramagnetic Manganese, pyrrhotite 



(1) Ferromagnetic Steel, magnetite, (with marked 



magnetic reactions) 



MAGNETS AND MAGNETISM 



Most magnets used in the instrument phase of magnetic prospecting are 

 parts of the magnetic systems of field magnetometers or are necessary 

 in the calibration of such instruments. These magnets are made of special 

 types of steel. A number of substances, such as chromium, tungsten, nickel, 

 cobalt, and the like, when added to iron in small yet critical percentages, 

 greatly affect its magnetic properties. Thus an alloy containing up to 4% 

 of nickel, tungsten, and cobalt, which has been quenched in cold water 

 after having been heated to 850° C, produces magnets of high retentivity 

 or great resistance to loss of magnetization. 



The metallurgy of steels for magnetic systems is an important feature 

 of instrument design ; a balance must be made between a number of factors 

 that relate to the composition of a particular steel, such as hardness, reten- 



* This matter is treated in detail under the subject of magnetic permeability of 

 materials. 



** This classification is based on the findings of Faraday that all materials respond in 

 one way or another to a magnetic field and therefore are diamagnetic or paramagnetic. 



