Electromotive Force of different Forms of ike Clark Cell. 119 



In examining: the resnlts it was noticed that the value of the co- 

 efficient did not depend on the quantity of crystals in the cell, but 

 on the distance of the end of the zinc rod from the crystals. The 

 difference of the results was, therefore, evidently due to slowness of 

 diffusion of the state of saturation through the body of the solution. 



So long ago as 1886 a set of five cells was set up by one of the 

 present writers at the Cavendish Laboratory, Cambridge, with zinc 

 rods of different lengths, with the object of testing the effect of 

 diffusion on the temperature-coefficient. These five cells were of the 

 original Rayleigh pattern, with platinum wires sealed through the 

 bottom. They could not be immersed in a water-bath, and were 

 found to be of a somewhat unsuitable form for the experiment, 

 which was then discontinued for more important work. Some more 

 recent tests of this set of cells will be found in the paper on the 

 Clark cell, by Glazebrook and Skinner.* The cells are numbered, 

 •6, 7, 8, 9, and 10 in the paper, and are among the oldest in the 

 possession of the Cavendish Laboratory. It will be seen from the 

 tests that they exhibit small discrepancies such as might very 

 probably arise from differences in the time required for diffusion in 

 the different cells. From a study of the above-mentioned paper, it 

 appears likely, in our opinion, that the differences observed in the 

 ease of the other cells of the same type may have been affected by a 

 similar cause. 



It would appear, in fact, inevitable that cells of the simple Board 

 of Trade pattern should exhibit some effects of diffusion-lag, 

 especially if subjected to considerable or rapid changes of tempera- 

 ture. This form of cell, however, is so convenient to use, and so 

 easy to make, that it seemed to us desirable to make a more careful 

 examination of the case, with the object, if possible, of constructing 

 ■cells of this form with a definite temperature-coefficient and a negli- 

 gible diffusion-lag. 



In October, 1894, the class was joined by Mr. H. T. Barnes, who, 

 as assistant to Dr. Harrington, Professor of Chemistry, had obtained 

 considerable experience in chemical manipulation. From this date 

 onwards the work of making and testing various forms of Clark 

 eell has been performed almost entirely by Mr. Barnes, but the 

 observations and calculations throughout .have been checked and 

 verified by Professor Callendar, who has devoted special attention to 

 the thermometry. 



§ 3. Constant-temperature Baths. 



The first step in the investigation consisted in making a pair of 

 suitable water-baths, controlled by delicate thermostats, which could 



* 'Phil. Trans.,' A, vol. 183 (1892), p. 586. 



