18 SECTIONAL ADDRESSES. 



General Character of Earth's Magnetic Field. 



The fundamental problem of terrestrial magnetism is the cause of the 

 earth's magnetism, and a secondary one the cause of the variations. Let 

 us consider the facts. Measurements of the intensity, declination and 

 inclination of the earth's magnetic field at thousands of stations distributed 

 over practically the whole of the earth's surface, show that the magnetic 

 field is roughly that of a uniformly magnetised sphere with its axis inclined 

 at about 12° to that of the earth's axis of rotation. An analysis of results 

 leads to the conclusion that the earth's uniform field is equivalent to that 

 produced by a magnetic doublet at the centre of the earth, the magnetic 

 moment of the doublet being 8.04 X 10'^' C.G.S. units. If I is the intensity 

 of magnetisation of the sphere and r the radius of the earth, the magnetic 



moment is given by- tc/I. 



Examination of the observations shows also that the intensity of 

 magnetisation of the earth is slowly changing, and that the earth's 

 magnetic field is slowly moving from east to west, the magnetic poles 

 performing circular or spiral motions about the geographical poles. In 

 addition to this secular change, there are yearly and daily changes, which, 

 although fairly constant in character, are changeable in magnitude. 

 There are also at times very violent changes in the magnetic elements, 

 such changes being known as magnetic storms. 



In an attempt to find the cause of the earth's magnetism, it is natural 

 to consider all the possible ways by which the earth can function as a 

 magnet, and even if we have to assume knowledge of the constitution of 

 the earth's interior, or assume the existence of electric currents, or assume 

 some physical state in the earth or in the atmosphere above it, in order to 

 account for the phenomena, we must not cast aside such assumptions, 

 unless it can be shown that they are imnecessary, and until data are 

 obtained which show the premises to be false. 



The first and simplest theory is that the earth is a large permanent 

 magnet, due to the magnetisation of the material of which it consists. 

 Another hypothesis is that the electric charge on the earth's surface 

 produces the magnetic field by its rotation. A third possibility is that 

 the earth is an electromagnet, the magnetising currents being either 

 outside the earth or within it ; such a system was roughly illustrated by 

 Wilde's magnetarium. Other theories postulate the field to be due 

 almost entirely to electric currents circulating within the earth or to be 

 due to the rotation of the earth. 



Fortunately, very valuable criteria have been given by Gauss and by 

 Schuster, the former showing that the main origin of the earth's magnetic 

 field is within the earth, and the latter that the cause of the daily variations 

 is external to the earth's surface. Any predominant magnetic effect 

 due to external causes need not, therefore, be looked for. 



In the first place, it is well to consider the main or permanent field, 

 i.e. that part of the magnetic field due to causes within the earth. 



Gauss, in his Memoir on Terrestrial Magnetism, expressed the magnetic 

 potential of the surface field of the earth in a series of spherical harmonics. 

 Some time afterwards, Everett pointed out that the first term of this 



