1892.] The Clark Cell as a Standard of Electromotive Force. 61 



An account of nine separate experiments is given in the paper ; the 

 following are the results reduced to 15° C. : — 



No. of 



E.M.F. of 



No. of 



E.M.F. of 



experiment. 



cell. 



experiment. 



cell. 



1 



1 -4341 



6 



1 -4342 



2 



1 -4336 



7 



1 -4342 



3 



1 -4341 



8 



1 -4340 



4 



1 '4340 



9 



1 4345 



5 



1-4340 







The mean of these is 1*4341, or, correcting for the rate of the clock, 

 1-4342. 



In Experiment. 2 the current in the voltameter was rather un- 

 steady, which may account for the low value ; while in Experiment 9 

 the temperature of the cell was changing somewhat, and our later 

 experience has shown us that the E.M.F. in our standard cell lags 

 very considerably behind the temperature. Still even taking these 

 experiments into account, the results are very close. 



If we suppose, as seems most probable, for reasons given in the 

 paper, that our cell is No. 4 of Lord Rayleigh's paper, and that it has 

 retained relative to No. 1 (Lord Rayleigh's standard) the value it had 

 in 1883, the E.M.F. of his cell No. 1 would be in the units he used 



1-4346 volts at 15°. 



The value found by Lord Rayleigh was 1*4348 volts; thus the two 

 are very close. 



In the units we have given above, those specified by the Board of 

 Trade, we have finally the result that the E.M.F. of our cell is 



1-4342 volts at 15° C. 

 or 1-4324 volts at 62° F. 



Paet II. 



In the second part of the paper we have investigated some of the 

 sources of error in the Clark cell, and also the effects of small varia- 

 tions in the materials used and the method of their preparation. We 

 have also compared a number of cells set up by different makers. 

 The general result is a very good agreement among cells from very 

 various sources. 



Cells set up by Lord Rayleigh in 1883 and 1884, Mr. Elder in 

 1886, Mr. H. L. Callendar in 1886, Dr. Muirhead in 1890, and by Dr. 



