AXI) MODEllX PHYSICS. 193 



tho inoclmnical forces acting on tho conductors carry- 

 ing the currents, are duo to this state of motion, or it 

 may be of strain, in the ether. It must not be sup- 

 posed that tho term electric displacement in Maxwell's 

 mind meant an actual bodily displacement of the 

 particles of tho ether; it is in some way connected 

 with such a material displacement. In his view, with- 

 out motion of the ether particles there would be no 

 electric action, but he does not identify electric 

 displacement and tho displacement of an ether 

 particle. 



His mechanical theory, however, does account for 

 tho electro-magnetic forces between conductors carry- 

 ing currents. The energy of the system depends on 

 the relative positions of the currents which form part 

 of it. Now, any conservative mechanical system 

 tends to set itself in such a position that its potential 

 energy is least, its kinetic energy greatest. The 

 circuits of the system, then, will tend to set themselves 

 so that the electro-kinetic energy of the system may 

 be as large as possible; forces will be needed to hold 

 them in any position in which this condition is not 

 satisfied. 



We have another proof of tho correctness of the 

 value found for the energy of tho field in that the 

 forces calculated from this value agree with those 

 which arc determined by direct experiment 



Again, the forces applied at the various driving- 

 points are transmitted to other points by the con- 

 nections of tho machine ; the connections are thrown 

 into a state of strain ; stress exists throughout their 

 substance. When we see the piston-rod and the shaft 



