404 



H. S. Carhart — Standard Clark Cell. 



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

 The mercnrous sulphate is only slightly soluble in a saturated 

 solution of zinc sulphate. I prevent local action, therefore, 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. C. 



Observed. 



Calculated. 



8-3 



1-0108 



1-0106 



5°1 



1-0124 



1-0125 



8-5 



1-0103 



1-0105 



10-6 



1-0106 



1-0103 



9-3 



1-0104 



1-0102 



125 



1-0098 



1-0096 



11-8 



1-0093 



10092 



15-2 



1-0087 



1-0086 



138 



1-0084 



1-0085 



195 



1-0069 



1-0069 



15-0 



1-0080 



1-0080 



21-2 



1-0062 



1-0062 



181 



10069 

 1-0064 



1-0068 

 1-0063 



31-1 



1-0024 



1-0024 



19-4 









19-9 



1-0062 

 1-0060 



1-0061 

 1-0059 





No. 113. 





20*3 









20-8 



10054 



1-0057 



5-1 



1-0124 



1-0125 



21-1 



1-0057 



1-0056 



10-6 



10106 



1-0104 



21-6 



1-0054 



1-0055 



12-5 



1-0098 



1-0097 



22-4 



1-0050 



1-0052 



15-2 



1-0088 



1-0087 



23-3 



1-0048 



1-0048 



19-5 



10070 



1-0070 



25-1 



1-0044 



1-0041 



21-2 



1-0062 



1-0063 



26-4 



1-0035 



1-0036 

 10022 



31-1 



1-0025 



1-0025 



30-2 



1-0019 









331 



1-0014 



1-0013 









39-1 



0-9991 



0-9989 









41 -1 



0-9980 



0-9979 









50-4 



09949 



0-9947 









52 7 



0-9939 



0-9940 









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

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

 reduction coefficient, '00077 per degree C. 



The equation for the E. M. F., derived from the observa- 

 tions on No. 17, is 



E'=E[1 — '000387(£ — 15) + -0000005^— 15) 2 ]. 



The calculated values for the three cells were all obtained 

 by this formula. The change for one degree C. is, then, the 

 following linear function of the temperature : — 



— •000386 + -000001 (*— 15). 



The temperature-coefficient ranges from "000361 at 0° C. to 

 •000376 at 25° C, and to -000361 at 40°C. At the highest ob- 

 served temperature in the preceding table it was only 000348. 

 The curve of E. M. F. with temperatures as abscissas is clearly 

 concave upward, indicating a fall in the temperature-coefficient 



