THE EARTH AND SUN AS MAGNETS 



107 



as it may appear at present, may yet prove to be true. Is the sun a magnet? 



Summing up the situation in April, 1912, he repeated: 



The evidence (whether the sun is a magnet) rests entirely on the form of 

 certain rays of the corona, which — assuming that they indicate the path of 

 projecting particles — seem to be deflected as they would be in a magnetic field, 

 but this evidence is not at all decisive. 



There remained the possibility of an appeal to a conclusive test of 

 magnetism: the characteristic changes it produces in light which orig- 

 inates in a magnetic field. 



Before describing how this test has been applied, let us rapidly re- 

 capitulate some of the principal facts of terrestrial magnetism. You see 

 upon the screen the image of a steel sphere (Fig. 2), which has been 



Fig. 2. Lines of Foecb of a Magnetized Steel Sphere. 



strongly magnetized. If iron filings are sprinkled over the glass plate 

 that supports it, each minute particle becomes a magnet under the influ- 

 ence of the sphere. When the plate is tapped, to relieve the friction, the 

 particles fall into place along the lines of force, revealing a characteristic 

 pattern of great beauty. A small compass needle, moved about the 

 sphere, always turns so as to point along the lines of force. At the 

 magnetic poles, it points toward the center of the sphere. Midway be- 

 tween them, at the equator, it is parallel to the diameter joining the 

 poles. 



As the earth is a magnet, it should exhibit lines of force resembling 

 those of the sphere. If the magnetic poles coincided with the poles of 

 rotation, a freely suspended magnetic needle should point vertically 



