﻿746 Unilateral Conductivity of Sal piloted Accumulator Plates, 

 following table, are approximately constant : — 

 Current. \ Vdt, 



•73 



638 



•81 



734 



•90 



696 



1-11 



803 



1*43 



646 



1-60 



785 



2-36 



720 





Mean value 717 



In all the above experiments the current was sent in that 

 direction for which the negatively formed plate is the anode. 

 When the p.d. is applied in the reverse direction, the curve 

 obtained for the decrease of V is similar in type, but the 

 initial value, V , is much lower, the reading is very unsteady, 

 and visible sparking occurs at the cathode, apparently across 

 the resisting film. The cell thus possesses, temporarily, the 

 property of unilateral conductivity, offering a much larger 

 resistance to a current sent outwards from the ;t formed" 

 plate than to one in the reverse direction. The gas-layer 

 theory developed by Schulze * for the corresponding property 

 of the aluminium anode-film involves assumptions which are 

 not possible in this case, and the more recent suggestion, of 

 Corbino and Maresca f, of a perforation of the film of 

 metallic salt is also unsatisfactory. The effect persists in the 

 absence of any electrolyte ; as, for example, in the case where 

 the dry cathode with its film is pressed against a clean sheet 

 of metal, and a current passed between the two. It is 

 possible that the original electric deposition of the film 

 involves a directional property in the substance of the film, 

 which would not appear if the lead sulphate were chemically 

 deposited ; but, unfortunately, it has not been found possible 

 to obtain a film deposited by chemical means alone to test 

 this point. 



A physical basis may be found for the assumption we have 

 made regarding the rate of breaking down of the film, for 

 we may consider that breaking clown as a mechanical effect 

 produced by the passage of electrons through the film. This 

 effect would be proportional to the velocity of the electrons, 

 and hence to the potential difference across the film, producing 

 this velocity. 



Physical Department, 



University College, London. 



* Schulze, Wied. Ann. xxi. p. 929 (1906). 



f Corbino & Maresca, Electrician, Dec. 28, 1906. 



