410 On a neiv Theory of Terrestrial Magnetism. 



the writers have failed to realize that the various parts of a 

 rotating sphere have relative motions one to the other, of such 

 a nature that if some were electrified the others would become 

 magnetized. Their difficulty seems to have arisen from the 

 fact that the motion of a particle of a rotating rigid sphere 

 consists of a rotation round the axis of the sphere com- 

 bined with a rotation of the particle round its own axis ; so 

 that if two particles be looking at one another in one position 

 of the sphere, they are looking at one another in all positions, 

 just as the same side of the moon is always turned to the earth. 

 But if this rotation of the particles round their own axes could 

 be stopped, if, in fact, the motion of the particles became 

 what is commonly known as " sun-and-planet motion " similar 

 to that of the bobbins in the machinery used in sheathing tele- 

 graph-cables to prevent torsional strain being put into the 

 iron wires as they are lapped on the core), then probably these 

 writers would have no difficulty in seeing that the interior 

 particles would be magnetized by the more rapidly moving 

 electrified ones. Now the motion of an tmelectrified particle 

 round its own axis cannot in any way prevent it becoming 

 magnetized by electrified particles revolving round it ; for 

 if it could, it would be equivalent to saying that, if the elec- 

 trified particles were at rest and the imelectrified one revolving, 

 the mere revolution of the latter would magnetize itself 

 oppositely to the way it would be magnetized if it were at rest 

 and the electrified ones only in motion — a result not only 

 without experimental proof, but one also highly improbable. 

 Consequently, if the particle has the two motions together (as it 

 has in a rigid revolving sphere), it will still become magnetized 

 if the surface of the sphere is electrified. 



In fact, so little can the motion of a mass of iron prevent 

 its becoming magnetized by a moving charge of electricity, 

 that it has been suggested to us, within the last few days, by 

 Mr. Gr. F. Fitzgerald, of Trinity College, Dublin, that pro- 

 bably a mass of iron would become magnetized by a static 

 charge of electricity if both had rapid absolute motion in space, 

 even although in the same direction in parallel lines and with 

 the same velocity — in fact, that relative motion is unnecessary. 

 If this assumption (which has not yet been experimentally 

 tested) be true, then not only will the mathematical analysis 

 required in the investigation of the problem contained in this 

 paper be much simplified, but, in addition the charge of elec- 

 tricity on the earth's surface necessary to produce, by its me- 

 chanical rotation, the earth's known magnetic moment will be 

 considerably less even than the charge calculated above. We 

 hope to put this idea of Mr, Fitzgerald's shortly to an experi- 



