CHAPTER XXIII 

 TERRESTRIAL MAGNETISM 



The direction of the earth's lines of force at a given place To find 

 the declination To find the dip or inclination The earth inductor 

 The intensity of the field To determine the horizontal intensity H 

 The vibration experiment The deflection experiment Recording 

 instruments Results Sketch of Gauss's theory of terrestrial magnetism. 



Is* many magnetic and electric measurements it is necessary to know 

 the intensity and direction of the earth's magnetic field at the 

 place of experiment. It is also of the highest scientific interest to 

 determine the field of the earth as a whole, in the hope that we 

 may answer the questions how and where is the earth magnetised 

 and what is the origin of the magnetisation ? We shall give here 

 a brief account of the methods usually adopted to obtain the field 

 at a given place, a summary of the results of observations made at 

 the various observatories distributed over the earth's surface, and 

 of the progress so far made in the investigation as to where the 

 magnetisation resides and how the magnetic field is disturbed. As 

 to the origin we have at present no theory of certain value.* 



The direction of the earth's lines offeree at a given 

 place. It is convenient to describe the direction of the earth's field 

 at a given place by two angles : (1) The angle which the vertical 

 plane through the line of force the plane of the magnetic meridian 

 makes with the vertical plane through the geographical North arid 

 South the plane of the geographical meridian. This angle is called 

 the Declination. (2) The angle between the line of force and 

 the horizontal plane. This angle is called the Dip or Inclination. 



To find the declination. Let us suppose that we have a 

 compass needle with axis of figure coinciding with the magnetic 

 axis balanced without friction on the point C, Fig. 222. If the poles 

 are w, and if the earth's total intensity is I, we have two 

 equal and opposite forces ml, one acting at each end. These 

 may be resolved into horizontal components mH, and into 

 vertical components mV. The latter will give a couple rV/, 

 * A full account of the Kew instruments will be found in Gordon's Electricity 

 inn! M<i</iK<tixt, vol. i. chaps, x'v. and xv. ; or in Ency. Brit., 9th ed., vol. xvi. 

 p. 159, 'Meteorology, Terrestrial Magnetism. The results are discussed in that 

 article, and in "Magnetism, Terrestrial," 10th ed., vol. xxx. p. 453. In the 

 llthed. the corresponding articles are under "Inclinometer," " Magnetograph," 



.rnetometer," and " Magnetism, Terrestrial." See also Bauer's Land, Magnetic 

 Observations, Carnegie Institution, 1912. 



301 



