ON FARADAY'S LINES OF FORCE. 181 



is found to be physically identical with the potential in statical electricity, and 

 thus we have the means of connecting the two sets of phenomena. If we knew 

 what amount of electricity, measured statically, passes along that current which 

 we assume as our unit of current, then the connexion of electricity of tension 

 with current electricity would be completed*. This has as yet been done only 

 approximately, but we know enough to be certain that the conducting powers of 

 different substances differ only in degree, and that the difference between glass 

 and metal is, that the resistance is a great but finite quantity in glass, and a 

 small but finite quantity in metal. Thus the analogy between statical electricity 

 and fluid motion turns out more perfect than we might have supposed, for there 

 the induction goes on by conduction just as in current electricity, but the quan- 

 tity conducted is insensible owing to the great resistance of the dielectrics t. 



On Electro-motive Forces. 



When a uniform current exists in a closed circuit it is evident that some 

 other forces must act on the fluid besides the pressures. For if the current 

 were due to difference of pressures, then 'it would flow from the point of 

 greatest pressure in both directions to the point of least pressure, whereas in 

 reality it circulates in one direction constantly. We must therefore admit the 

 existence of certain forces capable of keeping up a constant current in a closed 

 circuit. Of these the most remarkable is that which is produced by chemical 

 action. A cell of a voltaic battery, or rather the surface of separation of the 

 fluid of the cell and the zinc, is the seat of an electro-motive force which 

 can maintain a current in opposition to the resistance of the circuit. If we 

 adopt the usual convention in speaking of electric currents, the positive current 

 is from the fluid through the platinum, the conducting circuit, and the zinc, 

 back to the fluid again. If the electro-motive force act only in the surface of 

 separation of the fluid and zinc, then the tension of electricity in the fluid 

 must exceed that in the zinc by a quantity depending on the nature and 

 length of the circuit and on the strength of the current in the conductor. 

 In order to keep up this difference of pressure there must be an electro-motive 

 force whose intensity is measured by that difference of pressure. If F be the 

 electro-motive force, / the quantity of the current or the number of electrical 



* See Exp. Bet. (371). t Exp. Res. VoL ra. p. 513. 



