TRANSACTIONS OF SECTION A. 159 



(i) that the quantities wliich define it are propagated through the aether with 

 a single constant velocity, and (ii) that translatory uniform convection as a whole 

 through the iether produces no modification in the field, then it is necessarily 

 restricted to the special type of the electrodynamic field as formulated by 

 Maxwell. 



(2) A field of gravitation is included as the limiting form of such a type 

 when the velocity of propagation becomes very great. As like source is now 

 to attract like, the energy of the field must be kinetic and not elastic. The 

 question of interaction between a field of gravitation and electric fields or rays 

 of light is, of course, a separate and fundamental one, independent of theories 

 of relativity, and is now being put to refined test by astronomical observation. 



(3) If time were linked with space after the maimer of a fourth dimen- 

 sion, relativity in electrodynamic fields would be secured as above, but the 

 sources of the field could not be permanent particles or electrons. If physi- 

 cal science is to evolve on the basis of relations of permanent matter and its 

 motions, time must be maintained distinct from space, and the effect of con- 

 vection must continue to be thrown on to the material observing system in the 

 form of slight modification of its structure. 



6. How could a Rotating Body such as the Sun become a Magnet? 



By Sir Joseph Larmor. 



The obvious solution by convection of an electric charge, or of electric polar- 

 isation is excluded ; because electric fields in and near the body would be 

 involved, which would be too enormous. Direct magnetisation is also ruled out 

 by the high temperature, notwithstanding the high density. But several feasible 

 possibilities seem to be open. 



(1) In the case of the sun, surface phenomena jjoint to the existence of a 

 residual internal circulation mainly in meridian planes. Such internal motion 

 induces an electric field acting on the moving matter : and if any conducting 

 path around the solar axis happens to be open, an electric current will flow round 

 it, which may in turn increase the inducing magnetic field. In this way it 

 is possible for the internal cyclic motion to act after the manner of the cycle 

 of a self -exciting dynamo, and maintain a permanent magnetic field from insigni- 

 ficant beginnings, at the expense of some of the energy of the internal circula- 

 tion. Again, if a sunspot is regarded as a superficial source or sink of radial 

 llow of strongly ionised material, with the familiar vortical features, its strong 

 magnetic field would, on these lines, be a natural accompaniment : and if it 

 were an inflow at one level compensated by outflow at another level, the flatness 

 and vertical restriction of its magnetic field would be intelligible. 



(2) Theories have been advanced which depend on a hypothesis that the 

 force of gravitation or centrifugal force can excite electric polarisation, which, 

 by its rotation, produces a magnetic field. But, in order to obtain sensible 

 magnetic effect, there would be a very intense internal electric field such as no 

 kind of matter could sustain. That, however, is actually got rid of by a 

 masking distribution of electric charge, which would accumiilate on the surface, 

 and in part in the interior where the polarisation is not uniform. The circum- 

 stance that the two compensating fields are each enormous is not an objection; 

 for it is recognised, and is illustrated by radioactive phenomena, that molecular 

 electric fieids are, in fact, enormous. But though, the electric masking would 

 be complete, the two distributions would not compensate each other as regards 

 the magnetic effects of rotational convection : and there would be an outstanding 

 magnetic field comparable with that of either distribution taken separately. 

 Only rotation would count in this way ; as the effect of the actual translation, 

 along with the solar system, is masked by relativity. 



(3) A crystal possesses permanent intrinsic electric polarisation, because its 

 polar molecules are orientated : and if this natural orientation is pronounced, 

 the polarisation must be nearly complete, so that if the crystal were of the 

 size of the earth it would produce an enormous electric field. But, great or 

 small, tliis field will become annulled by masking electric charge as above. The 

 explanation of pyi-o-electric phenomena by Lord Kelvin was that change of 

 temperature alters the polarisation, while the masking charge has not had 



