Electromotive Force of different Forms of the Clark Cell, 13U 



The electrical conditions are not less important, but are more easily 

 realised and maintained. Our galvanometer is sensitive to much less 

 than a millimetre of the bridge-wire (one hundredth of a millivolt) 

 •Great attention is paid to the perfection of the insulation, and to the 

 avoidance of thermo-electric effects, which may readily amount to 

 more than ten microvolts. We may here remark that in testing any 

 new batch of cells it is quite impossible to tell, till the results are 

 worked out, whether they are in agreement with others. The many 

 coincidences found cannot therefore be the result of bias on the part 

 of the observer. 



We think we may fairly claim for the Clark cell an order of con- 

 sistency approaching one-hundredth of a millivolt in the temperature 

 changes of its E.M.F. 



§ 20. Clark Cells in ivhich the Solution is of Constant Strength. 



It is well known that Clark cells, not containing crystals, in which 

 the solution does not change its strength with change of temperature, 

 have the advantage of possessing a temperature-coefficient which is 

 less than half that of the saturated cells. They are also practically 

 free from the effects of diffusion-lag, as the density of the solution is 

 always nearly uniform. 



The best known cell of this type is the Carhart- Clark cell, in which 

 the zinc sulphate solution is chosen as being saturated at 0° C. An 

 ■error of 2° C. in the temperature at which the solution is saturated, 

 will make an error of only one millivolt, approximately, in the E.M.F. 

 of the cell. 



We have prepared several cells of this type at different dates and 

 in different forms, with separately prepared solutions. In cells so 

 prepared, of similar patterns, we have not as a rule found differences 

 greater than two or three-tenths of a millivolt. These differences 

 were probably due as much to other cause as to difference of strength 

 of solution. We have generally sealed the cells hermetically to avoid 

 creeping of the solution, which has a tendency to lower the E.M.F. in 

 the case of unsaturated cells. 



It is evident that these cells must, on the whole, be less accurately 

 reproducible than the saturated cells. We have also found that they 

 •are more liable to undergo slight changes of E.M.F. as a result of 

 short-circuiting, or of exposure to high or low temperatures. They 

 appear, in fact, to be less stable than the cells containing crystals. 



We have also prepared experimental cells with solutions weaker 

 than the cell saturated at 0° C. We have observed in these cells a 

 .similar instability, becoming more marked as the solution is weakened. 

 We are inclined to attribute this instability to a difference in solubility 

 or diffusivity of the mercurous sulphate in the weaker solutions. 



