586 



MAGNETIC FIELD. 



of the deflecting bar orr either side of the movable magnet, and of 

 two reversals, so as to get rid of want of symmetry. 



In each case, the tangent of deflection was equal, within a cor- 

 rection, to the ratio of the field of the deflecting magnet to the 

 terrestrial field. The deflections 8 and 5' for the distance R, for 

 two different arrangements, satisfy very well the equations 



tan 8 = o-o8687oR~ 3 - 0-002 185 R~ 5 , 

 tan 8' = 0-043435 R~ 3 + 0-002449 R~ 5 > 



as is seen from the following tables : 



For a great distance, the ratio of the tangents of the deflections 

 that is, of the fields of the deflecting bar on the line of the poles, and 

 in the plane of the equator is equal to the ratio of the first co- 

 efficients, which is exactly 2. 



The law of the square of the distances is thus established directly 

 with a degree of exactitude which the experiments of Coulomb would 

 not furnish. 



1159. TERRESTRIAL FIELD. DECLINATION. The value of the 

 terrestrial field (305) is usually determined by the declination and 

 the inclination, which give the directions, and by the value of one 

 component. It would also be sufficient to know the declination and 

 two of the components. 



In order to have the declination, the geographical meridian must 

 first be determined, and then the azimuth in which the magnetic axis 



