from Faraday to J. J. Thomson. 375 



equivalent of ion.* Thus mE (u + v) = total current = k = raA, 

 or A = E (u + v). The determination of v/u by the method of 

 Hittorf, and of (u + v) by the method of Kohlrausch, made it 

 possible to calculate the absolute velocities of drift of the ions 

 from experimental data. 



Meanwhile, important advances in voltaic theory were 

 being effected in connexion with a different class of investi- 

 gations. 



Suppose that two mercury electrodes are placed in a solution 

 of acidulated water, and that a difference of potential, insufficient 

 to produce continuous decomposition of the water, is set up 

 between the electrodes by an external agency. Initially a 

 slight electric current the polarizing current,f as it is called 

 is observed; but after a short time it ceases; and after its cessation 

 the state of the system is one of electrical equilibrium. It is 

 evident that the polarizing current must in some way have set 

 up in the cell an electromotive force equal and opposite to the 

 external difference of potential ; and it is also evident that the 

 seat of this electromotive force must be at the electrodes, which 

 are now said to be polarized. 



An abrupt fall of electric potential at an interface between 

 two media, such as the mercury and the solution in the present 

 case, requires that there should be a field of electric force, of 

 considerable intensity, within a thin stratum at the interface > 

 and this must owe its existence to the presence of electric 

 charges. Since there is no electric field outside the thin stratum, 

 there must be as much vitreous as resinous electricity present ; 

 but the vitreous charges must preponderate on one side of the 

 stratum, and the resinous charges on the other side ; so that 

 the system as a whole resembles the two coatings of a con- 

 denser with the intervening dielectric. In the case of the 



* i.e. E is 96580 coulombs. 



t The phenomenon of voltaic polarization was discovered by Hitter in 1803. 

 Hitter explained it by comparing the action of the polarizing current to that of a 

 current which is used to charge a condenser. Volta in 1805 put forward the 

 alternative explanation, that the products of decomposition set tip a reverse 

 electromotive force. 



