Theory of the Capillary Electrometer. 



331 



My position in relation to the problem was very different. I 

 wanted to make a capillary electrometer from the description given 

 in Lippmann's Theses. In order to get better results, I determined 

 by actual experiment what were the conditions of sensitiveness and 

 rapidity, and in doing this found out so much about the instrument 

 that the "einfachste denkbare Annahme," referred to by Hermann, 

 would not have commended itself to me. 



My paper on the " Time-Relations of the Capillary Electrometer " 

 was a condensed account of a small portion of the work done by me. 

 For various reasons I did not then enter into my views as to the 

 theory of the instrument, and will confine myself here to a statement 

 of them, which must be regarded as preliminary. 



Professor Hermann speaks of my theory having been empirically 

 obtained. I demur to that expression as open to misconstruction. 

 My working formula may rightly be called empirical, since it neglects 

 certain tsrms of the complete expression, which I have found to 

 neutralise each other in a suitably selected instrument, but my theory 

 of the time-relations of the capillary electrometer was founded upon 

 first principles and verified by experiments. 



My starting point was the fundamental fact that in the capillary 

 electrometer a mechanical effect is produced by an electrical cause. 

 But there are several links between the cause and the effect, and 

 a strong probability that each of them involves a time-function. 



They are shown in the following scheme : 



Poiseuille showed in 1846 that the flow of a liquid through a 

 capillary tube varies directly as the pressure. Of this I was not 

 aware till later, but it leads to precisely the same differential equa- 

 tion as that adopted by Hermann. 



Writing Q for the quantity of electricity, C for the constant of 

 capillarity, P for polarisation, and W for the work done, the sym- 

 bolical expression of the problem is 



/(Q/, C,, P t ) = 0(W,). 



