4M Prof. Thomson on Transient Electric Currents. 



the electro-statical capacity of the principal conductor, the 

 greater will be the whole quantity of water decomposed. Pro- 

 bably the best arrangement in practice would be one in which 

 merely a small ball or knob is substituted for a principal 

 conductor fulfilling the conditions prescribed above ; but those 

 conditions not being fulfilled, the circumstances would not be 

 exactly expressed by the formulae of the present communica- 

 tion. The resistance would be much diminished, and conse- 

 quently the whole quantity of water decomposed much increased, 

 by substituting large platinum electrodes for the mere points 

 used by Wollaston ; but then the oxygen and hydrogen separated 

 during the first direct current would adhere to the platinum 

 plates, and would be in part neutralized by combination with 

 the hydrogen and oxygen brought to the same plates respectively 

 by the succeeding reverse current ; and so on through all the 

 alternations of the discharge. In fact, if the electrodes be too 

 large, all the equivalent quantities of the two gases brought suc- 

 cessively to the same electrode will recombine, and at the end of 

 the discharge there will be only oxygen at the one electrode and 

 only hydrogen at the other, in quantities electro-chemically equi- 

 valent to the initial charge of the principal conductor. Hence 

 we see the necessity of using very minute electrodes, and of 

 making a considerable quantity of electricity pass in each dis- 

 charge, so that each successive alternation of the current may 

 actually liberate from the electrodes some of the gases which it 

 draws from the water. Probably the most effective arrangement 

 would be one in which a Leyden phial or other body of consider- 

 able capacity is put in connexion with the machine and discharged 

 in sparks through a powerful discharger, not only of great 

 electro-dynamic capacity and of as little resistance as possible 

 except where the metallic communication is broken in the elec- 

 trolytic vessel, but of great electro- statical capacity also, so that 

 all, or as great a portion as possible, of the oscillating electricity 

 may remain in it and not give rise to successive sparks across 

 the space of air separating the discharger from the soiKce of the 

 electricity. 



The initial effect of a uniform electromotive force in establish- 

 ing a current in a linear conductor may be determined by giving 



C and Q infinite values in the preceding formulae, and p- a finite 



value V, which will amount to supposing the potential at one 

 end of the discharger to be kept constantly at the value V, while 

 the potential at the other end is kept at zero. The formulae 

 suitable to this case, which is obviously a case of non -oscillatory 

 discharge, are (6) ; and from them we deduce 



y=^(l-e--A'). ..... (16), 



