496 Prof. H. F. Weber's Researches on 



\ denotes that length of measuring-wire which must be in- 

 serted as the part common to both galvanic circuits in order 

 to produce in the second circuit perfect compensation of the 

 electromotive force to be measured, w the resistance of unit 

 length of the measuring wire, W the total resistance of the 

 first circuit, and D the electromotive force of the compensa- 

 ting Daniell, the expression of the electromotive force to be 

 measured is 



The length X of the measuring-wire which must be inserted 

 at the time t in order to fully compensate by the Daniell's 

 element the electromotive force present at that time between 

 the two zinc electrodes is accordingly connected with the time 

 in such wise that 



x= 3 Ai *~^' _Br ^*']' for ' >1 ' 



and, for greater values of t, 



Hence it follows that, if for a series of equidistant moments 

 of time t , t l7 t f 2 . . . , at the production of compensation of the 

 electromotive forces corresponding to the concentrations of the 

 boundary layers, the lengths of measuring-wire inserted have 

 been found equal to X , \ l9 X 2 . . . , then must the quantity 



*&) 



very slightly rise with increasing time, and conse- 

 quently with increasing i, and pretty soon become constant. 

 The constant value which this quantity finally reaches is 



K£)=S M <> 



in which At=t 1 —t =zt 2 —t 1 =:t 3 — 1 2 , &c. With the aid of this 

 constant value (call it tj) the quantity of Fick's " constant of 

 diffusion " can be ascertained; we get 



k = Vz^-AZ 



LM 



7T 2 At 



Results of the Observations. — The first two series of observa- 

 tions were made in an apartment at the Physical Institute in 

 the course of October 1878. In consequence of the warming, 



