218 Profs. W. E. Ayrton and John Perry. [Mar. 21, 



The changes which occur in the electromotive force of a cell are 

 difficult to explain. They may be due to imparities in the metals, to 

 air condensed at metallic surfaces, to air and other gases dissolved in 

 the liquid, &c. They seem to be quite arbitrary, and they are often 

 very considerable. The authors have begun an investigation regard- 

 ing the causes of these peculiar effects. 



The discrepancies observable in the following comparisons may also 

 be due to the smallness of the electromotive forces of contact which 

 were measured separately, but they are in some part due to the unex- 

 plained changes which occur in the electromotive force of a cell. Let 

 C, Z, and L represent the copper, zinc, and liquid respectively of a 

 simple cell ; let L x and L 2 be the liquid in contact with the copper, 

 and the liquid in contact with the zinc of a Daniell's cell ; let CL be 

 the electromotive force of contact of C and L, and let CL be identical 

 with — LC. Taking the five cells of Table XI, and assuming that the 

 electromotive force of a contact of two substances as it exists in a cell 

 is equal to its electromotive force when measured in our apparatus, we 

 get the following results : — • 



I; Daniell with pure saturated copper sulphate and 

 nearly pure saturated zinc sulphate. 



BLx+El^+Cz+zc 



= -028-0 -033 + -358 + -750=1 103 



II. Daniell with distilled water and pure saturated 



copper sulphate. 



CLx+LJ^s+EZ + ZC 

 = -028 + -071 + 126 + '750=0 -975 



III. Daniell with very dilute zinc sulphate and 

 slightly impure saturated copper sulphate. 



CLx + LXo + I^Z + ZC 

 = + -063 + 177 + -750=0 -990 



IV. Simple cell, nearly pure saturated zinc sulphate. 



CL + LZ + ZC 

 = -0 113 + -358 + -750=0 "995 



V. Simple cell, distilled water. 



CL+LZ+ZC 



= -074 + 126 + 0-750=0 '950 "832 to -942 



increasing slowly. 



In every case the sum of the separate contact electromotive forces 

 is so nearly equal to the observed maximum electromotive force of the 



Observed EMF 

 of Cell. 



1 '068 to 1 -081, 

 increasing 

 slowly. 



-995 mean 

 from Table X. 



1 -010 Table IX. 



1 -ooo 



