On an Improved Standard Clark Cell. 



421 



old, I found that the zinc had been removed from the rod at 

 the surface of the liquid and had been deposited again upon 

 the rod at the surface of the mercury salt, in a solid frill round 

 the zinc. The copper wire in this cell became entirely de- 

 tached, partly because of the expansion upward of the marine 

 glue, which brought a severe strain upon the wire. 



The local action then increases the zinc sulphate in the cell at 

 the expense of the mercury sulphate and amalgamates the zinc 

 rod. I have become convinced by some experiments extend- 

 ing over several w 7 eeks that this substitution process goes on 

 only when the zinc is in contact with the solid mercury salt. 

 The mercurous sulphate is only slightly soluble in a saturated 

 solution of zinc sulphate. I therefore prevent local action by 

 keeping the zinc and the mercury salt out of contact. The 

 same device operates to raise the E.M.F. about 0*4 per cent. 

 The following Table exhibits the observed and calculated values 

 of the E.M.F. of cells No. 17, 112, 113 in terms of No. 1 

 (old style) at 20° C. :— 



No. 17. 



No. 112. 



Temp. C. 



Observed. 



Calculated. 



Temp. O. 



Observed. 



Calculated. 



o 



8-3 



1-0108 



1-0106 



5°1 



1-0124 



1-0125 



8-5 



1-0103 



1-0105 



106 



1-0106 



10103 



9-3 



10104 



1-0102 



12-5 



1-0098 



10096 



11-8 



10093 



1-0092 



15-2 



1-0087 



1-0086 



138 



1-0084 



1-0085 



19 5 



10069 



1-0069 



150 



1-0080 



1-0080 



21-2 



1-0062 



1-0062 



18-1 



1-0069 



1-006S 



31-1 



1-0024 



1-0024 



19-4 



10064 



10063 









199 



1-0062 



1-0061 





No. 113. 





20-3 



20-8 



10060 

 1-0054 



1-0059 

 1-0057 















21-1 



1-0057 



10056 



51 



1-0124 



1-0125 



21-6 



1-0054 



1-0055 



10-6 



1-0106 



10104 



22-4 



10050 



1-0052 



125 



1-0098 



1-0097 



23-3 



1-0048 



1-0048 



15-2 



1-0088 



1-0087 



251 



10044 



1-0041 



19-5 



10070 



1-0070 



26-4 



1-0035 



1-0036 



21-2 



10062 



1-0063 



30-2 



1-0019 



1-0022 



31-1 



1-0025 



1-0025 



331 



1-0014 



10013 









391 



0-9991 



0-9989 









41-7 



0-9980 



0-9979 









504 



0-9949 



0-9947 









52-7 



0-9939 



0-9940 



1 







Cell No. 1 was always very near 20° C, and the reduction 

 to that temperature was made by means of Lord Rayleigh's 

 reduction-coefficient, '00077 per degree C. 



