126 



Prof. H. L. Callendar and Mr. H. T. Barnes. 



cient time is allowed, and sufficient crystals are present, the higher 

 value is the more correct one to use ; bnt we are inclined to think 

 that many observers are not aware how extremely slow the process 

 of diffusion really is, and how considerable a time is required for the 

 attainment of the limiting value. 



§ 9. Board of Trade " Crystal" Cells. 



The defects of the ordinary B.O.T. form of Clark cell in this 

 respect of diffusion-lag have long been recognised, and various 

 methods have been proposed to remedy the disadvantages resulting 

 from it. Lord Rayleigh himself preferred the H-form of cell, in 

 which the zinc is buried under a layer of crystals. This form of 

 cell has been adopted by the Berlin Reichsanstalt, and has been 

 shown by Kahle to be practically free from the defects of the other 

 pattern. We have made several cells of the H-form, but we are 

 strongly of opinion that, besides being more difficult to make, they 

 are not so convenient to work with as the B.O.T. test-tube pattern. 



On considering the matter in the light of the preceding obser- 

 vations with regard to the great differences in the temperature- 

 coefficient produced by different degrees of immersion of the zinc 

 rod in the crystals, it occurred to us that the diffusion-lag of the 

 B.O.T. pattern might be entirely removed by a very simple- modi- 

 fication of procedure ; so simple, in fact, that it would seem scarcely 

 to deserve mention, were it not that of the many hundreds of 

 B.O.T. cells which we have seen and examined, not one has been 

 constructed in the manner to be described. 



The modification we have adopted in these cells, which we term 

 Board of Trade " crystal " cells, consists simply in filling the cell 

 above the mercurous paste with moist crystals, instead of with satu- 

 rated solution. Under these circumstances no part of the solution 

 can remain either supersaturated or unsaturated for any appreciable 

 time. 



We have subjected these and other cells to the severest tests, and 

 the most sudden variations of temperature, such as 0° C. to 25° or 

 30° C, and we find that the B.O.T. cells, when filled in this manner 

 with crystals, have no appreciable diffusion-lag, and are not sur- 

 passed in quickness by any other form. 



§ 10. Preparation of Crystal Cells. 



The procedure which we adopt in making these crystal cells 

 differs only in one or two small details from that prescribed in the 

 Board of Trade memorandum on the Clark cell. 



A stock solution with mercurous sulphate paste is prepared 



