146 ANNUAL KEPOET SMITHSONIAN INSTITUTION, 1913. 



throw some light on the problem. Its high temperature wholly pre- 

 cludes the existence of permanent magnets, hence any magnetism it 

 may exhibit must be due to motion. Its great mass and rapid linear 

 velocitj^ of rotation should produce a magnetic field much stronger 

 than that of the earth. Finally, the presence in its atmosphere of 

 glowing gases and the well-known effect of magnetism on light 

 should enable us to explore its magnetic field even at the distance of 

 the earth. The effects of ionization, probably small in the region of 

 high pressure beneath the photosphere and marked in the solar atmos- 

 phere, must be determined and allowed for. But with this important 

 limitation the sun may be used by the physicist for an experiment 

 which can not be performed in the best equipped laboratory. 

 Schuster, in the lecture already cited, remarked: 



The form of the corona suggests a further hypothesis which, extravagant 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 pro- 

 jecting 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 

 originates in a magnetic field. 



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

 recapitulate some of the principal facts of terrestrial magnetism. 

 You see upon the screen the image of a steel sphere (fig. 2), which 

 has been strongly magnetized. If iron filings are sprinkled over 

 the glass plate that supports it, each minute particle becomes a mag- 

 net under the influence 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 between 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 

 ()f rotation, a freely suspended magnetic needle should point ver- 

 tically downward at one pole, vertically upward at the other, and 

 horizontally at the equator. A dip needle, used to map the lines 

 of force of the earth, is shown on the screen. I have chosen for 

 illustration an instrument designed for use at sea^ on the non- 

 magnetic yacht Carnegie^ partly because the equipment used by 



' niustrated iu article on Thf- Earth's Magnetism, by Dr. Bauer, pp. 195-212 of this 

 volume. 



