24 SECTIONAL ADDRESSES. 



To overcome the difficulties of a surface charge Sutherland suggested 

 an equal but opposite charge concentrated at the Centre of the earth, 

 thus neutralising the electrostatic field due to the surface charge but 

 not the magnetic effect of the charges in motion. Later, he suggested 

 that an inequality in the distribution of the earth's atomic charges 

 might be a cause. If all the negative and positive electricity in 

 the earth were spread over two concentric spheres slightly different in 

 diameter, a combined magnetic field comparable with that of the earth 

 could be obtained. In such a case, as the charges are enormously great, 

 the difference in radii of the spheres would need to be but exceedingly 

 small to produce the desired intensity of field. The difficulties due to 

 the external electrostatic field and the magnetic field produced by the 

 translatory motion of the earth vanish, since there will be no such fields, 

 but, unfortunately, not only would such a field be symmetrical about the 

 axis of rotation, but at internal points near the surface the electric field 

 would be many millions of volts per centimetre. 



There are a number of variants of this idea of separated charges. 

 One is that the rotation of the earth brings about an electric polarisation 

 in the atoms perpendicular to the axis of rotation, such polarisation pro- 

 ducing a magnetic and also an electrostatic field. The direction of 

 magnetisation of the field is not, however, that actually observed on the 

 earth, the same difficulty presenting itself as that already considered with 

 the charged sphere. 



In 1891, and on several occasions since, Schuster has raised the 

 question whether every large rotating mass is not a magnet, and as far 

 back as 1891 he put forward the suggestion that the sun had a magnetic 

 field associated with it. 



Lord Kelvin in 1892 remarked : ' Considering probabilities and 

 possibilities as to the history of the earth from its beginning to the present 

 time, I find it unimaginable but that terrestrial magnetism is due to the 

 greatness and rotation of the earth. If it is true that terrestrial magnetism 

 is a necessary consequence of the magnitude and the rotation of the earth, 

 other bodies comparable in these qualities with the earth and comparable 

 also with the earth in respect of material and temperature, such as Venus 

 and Mars, must be magnets comparable in strength with the terrestrial 

 magnet, and they must have poles similar to the earth's north and south 

 poles on the north and south sides of their equators because their directions 

 of rotation as seen from the north side of the ecliptic are the same as that 

 of the earth.' 



This suggestion that every rapidly rotating body produces a magnetic 

 field leads to the consideration of the most promising of all such bodies, 

 namely the sun, the radius of which is much greater than that of the 

 earth, and, moreover, its atmosphere contains vapours which are self- 

 luminous and, therefore, give line spectra. The sjjectra produced have 

 been examined at the Mount Wilson Solar Observatory, and the magnetic 

 field of the sun has been revealed by the well-known Zeeman effect. 

 Moreover, the intensity of the field has been measured at different 

 atmospheric depths, and it has been found that the magnetisation decreases 

 rapidly with height of the solar atmosphere. This decrease leads to the 

 conclusion that the sun's magnetic field does not pass into outer space to 



