208 ON FARADAY'S LINES OF FORCE. 



(1) That two particles of electricity when in motion do not repel each other 

 with the same force as when at rest, but that the force is altered by a quantity 

 depending on the relative motion of the two particles, so that the expression for 

 the repulsion at distance r is 



(2) That when electricity is moving in a conductor, the velocity of the 

 positive fluid relatively to the matter of the conductor is equal and opposite to 

 that of the negative fluid. 



(3) The total action of one conducting element on another is the resultant 

 of the mutual actions of the masses of electricity of both kinds which are 

 in each. 



(4) The electro-motive force at any point is the difference of the forces 

 acting on the positive and negative fluids. 



From these axioms are deducible Ampere's laws of the attraction of 

 conductors, and those of Neumann and others, for the induction of currents. 

 Here then is a really physical theory, satisfying the required conditions better 

 perhaps than any yet invented, and put forth by a philosopher whose experi- 

 mental researches form an ample foundation for his mathematical investigations. 

 What is the use then of imagining an electro-tonic state of which we have 

 no distinctly physical conception, instead of a formula of attraction which we 

 can readily understand 1 I would answer, that it is a good thing to have 

 two ways of looking at a subject, and to admit that there are two ways of 

 looking at it. Besides, I do not think that we have any right at present to 

 understand the action of electricity, and I hold that the chief merit of a 

 temporary theory is, that it shall guide experiment, without impeding the 

 progress of the true theory when it appears. There are also objections to 

 making any ultimate forces in nature depend on the velocity of the bodies 

 between which they act. If the forces in nature are to be reduced to forces 

 acting between particles, the principle of the Conservation of Force requires 

 that these forces should be in the line joining the particles and functions of 

 the distance only. The experiments of M. Weber on the reverse polarity of 

 diamagnetics, which have been recently repeated by Professor Tyndall, establish 

 a fact which is equally a consequence of M. Weber's theory of electricity and 

 of the theory of lines of force. 



