422 On an Improved Standard Clark Cell. 



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

 tions on No. 17, is 



E / = E[l--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° 0., and to -000361 at 40° C. At the highest 

 observed temperature in the preceding Table it was only 

 •000348. The curve of E.M.F. with temperatures as abscissae 

 is clearly concave upward, indicating a fall in the temperature- 

 coefficient with rise of temperature. The change is, however, 

 so small as to be quite negligible within the range of tempe- 

 rature to which a normal element is subjected in practice. 

 Lord Hayleigh's cells show a change in the temperature- 

 coefficient directly the reverse of the above ; that is, the 

 coefficient increases by a very appreciable quantity with rise 

 of temperature. For his No. [36] the coefficient ranged from 

 •000556 at 0° C. to -00101 at 25° C, if his equation holds 

 true for the higher temperature. 



In making comparisons of E.M.F. I have used Lord 

 Bayleiglr's method, slightly modified, by means of which a 

 difference of one ten-thousandth part is observed directly and 

 with the greatest ease. In fact a difference of half that 

 amount is easily measured. A comparison of half a dozen 

 cells can be made in as many minutes without difficulty. 



As to polarization, these cells show none with external 

 resistance greater than 30,000 ohms. At 30,000 ohms the 

 polarization is just discernible ; and with 10,000 ohms it 

 amounts to only one ten-thousandth part in five minutes. 

 This fall in E.M.F. is less than the accidental differences 

 between different cells in general, and much smaller than the 

 almost unavoidable errors due to ignorance of the real tem- 

 perature of the cell. If the cell is not closed on less than 

 10,000 ohms resistance, and only for a few minutes, the 

 polarization may be entirely neglected. 



As indicating the uniformity attained, the following relative 

 values of the E.M.F. of six cells, only four days old, may be 

 given :— 9048, 9049, 9049, 9048, 9046, 9043. The last one 

 was still approaching the others when last observed. Six 

 cells of later construction gave the following relative values 

 when less than two days old:— 9182, 9182, 9182*5,9182, 

 9182, 9182-5. The two sets of numbers do not represent at 

 all the relative values of one set as compared with the other. 



