88 EXPLORATION GEOPHYSICS 



ment of the formulae involved will be omitted. The relation is, however, 

 similar to Gauss Position II, where at a maximum the couple is : 



couple = MM'/d' (37a) 



Fig. 22. — ^Two magnets in 

 Position III of Gauss: where the 

 magnetic needle is above the de- 

 flector magnet M'. 



TERRESTRIAL MAGNETISM 



The terrestrial phase of geomagnetics, or, to express it more simply, 

 earth magnetism, utilizes the fundamentals of magnetism which have been 

 presented. The earth itself acts like a great spherical magnet. In common 

 with other magnets the earth has the power of attraction, exhibits polarity, 

 and has a field of force. Except for its large size, which is not inherently 

 a magnetic characteristic, the earth acts quite like any ordinary spherical 

 magnet. 



It is advantageous to take up geophysically pertinent features of earth 

 magnetism before discussing the magnetic properties of rocks and forma- 

 tions in detail. This is because rock magnetization is considered to be 

 caused largely by induction in the earth's magnetic field. 



The general idea that the earth itself could be considered as a spherical 

 magnet, and so treated, was developed some 350 years ago by Sir William 

 Gilbert.* Gilbert carefully investigated the magnetic character of a sphere 

 (or terella) turned out of magnetite, by placing small magnets mounted on 

 pivots at various places on and around it. In his famous treatise De 

 Magnete published in the year 1600, he wrote, "The earth is a great 

 magnet." 



Gilbert spent 18 years in the study of magnetism. The great value of 

 his work lay in the fact that he thoroughly believed in the importance of 

 experimentation. With his spherical loadstone he obtained a very good 

 understanding of the distribution of the lines of force of the magnetic field 

 of the earth, and their inclination or dip. He studied the change from zero 

 dip at the equator to 90° dip at the poles and established that the rate of 



* See Chapter I, page 5. 



