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XLIX. An Improved Standard Clark Cell with Low 

 Temperature- Coefficient. By H. S. Carhakt.* 



THE best form of Clark cell hitherto made is that of 

 Lord Rayleigh, described in the ' Philosophical Trans- 

 actions ' for 1885. The objections to this form are that the 

 temperature-coefficient is not the same for all cells, as is shown 

 in Lord Rayleigh's paper, and it is so high as to introduce a 

 very troublesome and uncertain error because of the difficulty 

 of ascertaining the exact temperature of the cell ; secondly, it 

 is not so constructed mechanically as to prevent the mercury 

 from coming into contact with the zinc when the cell is sub- 

 jected to violent jars in transportation ; thirdly, a great 

 chemical defect is the facility with which local action takes 

 place between the zinc and the mercury salt. I might add 

 that the mercurous sulphate, purchased by Lord Rayleigh, 

 evidently contained considerable salt in the mercuric form, as 

 is shown by its turning yellow on mixing with the zinc- 

 sulphate solution. 



All these difficulties I have, at least in large measure, over- 

 come. Respecting the materials, the greatest care is required 

 to secure and maintain cleanliness and purity in their pre- 

 paration. The mercury must be distilled in vacuo after being- 

 cleaned by chemical means. The zinc sulphate should be free 

 from iron as well as other impurities. The mercurous sulphate 

 can be made almost or quite free from the mercuric form by 

 using plenty of mercury ; keeping the temperature down to 

 the lowest point at which action will take place ; and letting 

 the mixture of salt, acid, and metallic mercury stand for some 

 time. I have made in this way a salt that remains white, not 

 only when the free acid is all washed out, but when mixed 

 with the standard zinc-sulphate solution. Further, it remains 

 white in the cell indefinitely if it is not exposed to a bright 

 light. 



Hitherto the importance of the local action going on in a 

 Clark cell appears not to have been appreciated. It accounts 

 for some of the differences in temperature-coefficient, and leads 

 to some more serious results in some cells. The zinc replaces 

 mercury when in contact with the mercury salt. This amal- 

 gamates the zinc, producing a slight change in the E.M.F. ; 

 and then the amalgam is liable to creep up to the top of the 

 zinc, where it attacks the solder. The copper wire is thus 

 sometimes loosened. The zinc sulphate follows up,, and the 

 cell may be thus short-circuited by the zinc and the copper 

 wire. Upon taking down one cell, which was perhaps a year 

 * From an advance proof communicated by the Author. 



