The Followers of Maxwell. 351 



magnetism as the secondary effect, and to ascribe magnetic 

 fields, not to the presence of magnetic tubes, but to the motion 

 of electric tubes. In order to account for the fact that magnetic 

 fields may occur without any manifestation of electric force, he 

 assumed that tubes exist in great numbers everywhere in space, 

 either in the form of closed circuits or else terminating on atoms, 

 and that electric force is only perceived when the tubes have a 

 greater tendency to lie in one direction than in another. In a 

 steady magnetic field the positive and negative tubes might be 

 conceived to be moving in opposite directions with equal 

 velocities. 



A beam of light might, from this point of view, be regarded 

 simply as a group of tubes of force which are moving with the 

 velocity of light at right angles to their own length. Such a 

 conception almost amounts to a return to the corpuscular 

 theory ; but since the tubes have definite directions per- 

 pendicular to the direction of propagation, there would now 

 be no difficulty in explaining polarization. 



The energy accompanying all electric and magnetic pheno- 

 mena was supposed by Thomson to be ultimately kinetic energy 

 of the aether ; the electric part of it being represented by rota- 

 tion of the aether inside and about the tubes, and the magnetic 

 part being the energy of the additional disturbance set up in 

 the aether by the movement of the tubes. The inertia of this 

 latter motion he regarded as the cause of induced electromotive 

 force. 



There was, however, one phenomenon of the electromagnetic 

 field as yet unexplained in terms of these conceptions namely, 

 the ponderomotive force which is exerted by the field on a 

 conductor carrying an electric current. Now any pondero- 

 motive force consists in a transfer of mechanical momentum 

 from the agent which exerts the force to the body which 

 experiences it ; and it occurred to Thomson that the pondero- 

 motive forces of the electromagnetic field might be explained if 

 the moving tubes of force, which enter a conductor carrying a 

 current and are there dissolved, were supposed to possess 



