Mercury-dropping Electrodes. 387 



in the current. The limit to this increase occurs when the 

 current which can flow, taking into account the resistance of 

 the arrangement and the small electromotive force available, 

 combined with any purely local action at the surface of the 

 drop, has not time to effectively tarnish it before it breaks 

 away. 



11. It follows that in the experiment described in § 4 the 

 potential of the resting mercury remains constant, while that 

 of the dropping mercury falls with the increasing speed of 

 dropping till the maximum is reached, as described in § 10. 

 I give the following considerations as supporting this view 

 and as showing my difficulties in accepting that of von 

 Helmholtz. 



12. Since in § 3 the positive potential of the lower mercury 

 is said to be due to the concentration on its surface of the 

 films produced on the drops, and in § 4 that maximum 

 difference of potential occurs when the drops form and break 

 away before they have time to get charged at all, it follows 

 that with quick dropping, producing maximum difference of 

 potential, the alteration of mercury surface forming the re- 

 quired double layer must, according to von Helmholtz^s view, 

 take place during the fall of the drops through the liquid and 

 after they have separated from the upper funnel. 



With somewhat slower dropping the alteration would be 

 partly before separation and pai-tly during the fall. 



In either case the length of fall is a factor in the result, 

 and making it small or zero should at least decrease the 

 current and electromotive force of the arrangement. 



And, further, since the current is in both cases carried by 

 the falling drops and not by the electrolyte through which 

 they fall, any variation in the resistance of the electrolyte 

 should have no effect on the current. 



13. I have found, however, by experiment, that decreasing 

 the fall increases the current and has no effect on the electro- 

 motive force, while decreasing the resistance of the electrolyte 

 increases the current. 



A thistle- funnel drawn out to a fine tube whose orifice was 

 about '17 millim. diameter, and which delivered with a head 

 of 42 centim. of mercury (the whole length of the funnel) 

 about I cubic centim. per minute, was fixed with its point 

 just below the surface of a column of spring-water 5^ 

 centim. diameter by 33 centim. high, contained in a tall glass 

 over a little mercury at the bottom. 



The difference of potential of this bottom mercury and that 

 in the funnel, measured by a quadraut electrometer (giving 

 72 divisions for the E.M.F. of a Daniell's cell), was, with the 



2 (^2 



