]f ; &5.] Induction in the surrounding Field. 171 



energy, and the results suggest that a similar action occurs at the 

 source of energy or seat of E.M.F. in other cases. 



A Circuit containing a Voltaic Cell. 



The chemical theory of the cell is adopted. It is first shown that 

 the difference of potential of the two terminals in open circuit is 

 equal to the E.M.F. immediately after closure, because chemical action 

 will go on charging the terminals, i.e.,' putting out energy into the 

 medium, until any further charge would require more energy in the 

 medium than is supplied by the chemical action necessary, according 

 to Faraday's law of electrolysis. 



The level surfaces are discussed, and it is Supposed that the poten- 

 tials being in ascending order, zinc, copper, acid, all of the surfaces 

 pass between the zinc and the acid, some of them bending round and 

 passing between the copper and acid, the rest going between the 

 terminals. When the circuit is closed, tubes of electric induction, 

 running from acid to zinc, are supposed to diverge outwards and close 

 in on the rest of the circuit, there running in the opposite direction, 

 I.e., from copper to zinc. A divergence of negative tubes being 

 magnetically equivalent to a convergence of positive tubes, the mag- 

 netic relations of the circuit or the direction of the current will be the 

 same throughout. The tendency to a steady state is discussed. The 

 existence of charges in the circuit is taken to show that the tubes of 

 electric induction do not enter the wire at the same time throughout 

 the whole of their length. 



Current produced by Motion of a Conductor in a Magnetic Field. 



In this case it is shown that according to the hypothesis we must 

 suppose a divergence of negative tubes from the seat of E.M.F. 



The General Equations of the Electromagnetic Field. 



The assumption that if we take any closed curve, the number of 

 tubes of magnetic induction passing through it is equal to the excess 

 of the number which have moved in over the number which have 

 moved out through the boundary since the beginning of the formation 

 of the field, suggests a historical mode of describing the state of the 

 field at any moment. 



Taking L,M, N as the numbers of magnetic tubes which have cut unit 

 lengths of the axes through a point since the beginning of the system, 

 those being considered positive which tend to produce positive electric 



intensity, the equations a = c I^l — ^ an( j two others are obtained. 

 J 1 ^ dz dy 



From these and the ordinary current equations 4nr/xu = — — — , and 



dy dz 



n 2 



