22 SECTIONAL ADDRESSES. 



the system is obtained at the expense of the energy of the circulating 

 conducting material. 



While in the case of the earth any internal circulation of matter in 

 meridian planes or near thereto is entirely conjectural, the theory does 

 pro^dde not only for the main field but also for the secular variation by 

 changing the paths of the circulating currents. 



Eoss Gunn has recently put forward a theory attributing the magnetic 

 field to electrical currents set up inside the earth in the high temperature 

 regions where the thermal motions are considerable. Gunn suggests that 

 the temperature of the inner earth is of the order of 10,000° and as a con- 

 sequence the material will be highly ionised and the conductivity corres- 

 pondingly great. In the case of the upper atmosphere, Gunn has 

 analysed the motions of ions and electrons of long free path spiralling about 

 the magnetic lines of force, and in such a case a diamagnetic effect and 

 drift currents are produced. An extension of the calculation to the inner 

 earth where the free paths are short is made, and it is considered that the 

 primary current system of the earth results from the motions imposed 

 upon ions having a mean free path of the order 10"' cm., the motion being 

 imposed by the internal gravitational electric field at right angles to the 

 magnetic field. The currents produced augment the original field in a 

 regenerative manner. Gunn is developing this theory of internal ion 

 drifts. 



Magnetic Effects Associated with Earth's Rotation. 



If the earth's magnetism were due to an iron core permanently 

 magnetised, or to electric currents circulating round the earth, the 

 rotation of the earth would play no part in the main or permanent field, 

 although, as will be seen later, it may contribute to variations due to 

 external causes. 



Let us consider the possible ways a body may, by virtue of its rotation, 

 act like a magnet. First, consider the earth as a body carrying a positive 

 or negative electric charge. Maxwell first of all suggested the production 

 of a current by the motion of an electric charge, and Rowland first demon- 

 strated the effect. Ayrton and Perry, over 50 years ago, showed that an 

 electric charge residing on the surface of the earth would produce a 

 magnetic field by the rotary motion of the earth, but to produce a field of 

 the requisite intensity the charge must be very great. If the surface density 

 of the charge be p the magnetic force at the equator parallel to the surface is 



H, = -7rp?co 



where w is the angular velocity and r the radius of the earth. If Q is the 

 total charge on the surface the horizontal magnetic force may be written 



H, = QW3r = ^ 



where V is the potential In this case it is obvious that any small sphere | 

 charged at the same potential and rotating at the same angular velocity 

 would produce the same surface field, since the radius of the sphere is not 

 involved. 



