41 8 MAGNETIC CONDITION OF THE GLOBE. 



A star, whatever may be the distribution of its magnetism, is, in 

 fact, equivalent for very distant points to an infinitely small magnet, 

 or to a sphere magnetised uniformly. 



Let us denote by : 



I the mean intensity of the Earth's magnetisation ; 



R its radius ; 



I' the mean intensity of the magnetisation of a star ; 



R ; its radius ; 



7' its magnetic moment ; 



D its distance from the Earth. 



The value of the action of the Earth at the equator, where it is 

 a minimum, is (153) 



If we suppose that the line of the poles of the star in question is 

 'directed towards the Earth, which is the most favourable case, the 

 force F^, which it will exert on the Earth, will be (153) 



The ratio of the polar action of the star in question, to the 

 equatorial action of the Earth, is 



D 



The ratio is therefore proportional to the magnetisation of the star 

 and to the cube of its apparent diameter. 



The apparant diameter of the Sun, and that of the Moon, are 

 about 30' that is to say, less than o.oi so that we have 



FP 1 1' _ 6 



