H. S. Carhart — Standard Clark Cell. 



403 



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. ; 



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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 thus be short-circuited by the zinc and the copper 

 wire. [Jpon taking down one celi, which was perhaps a year 

 old, I found that the zinc had been removed from the rod at 

 the surface of the liquid and had been deposited again upon 

 the rod at the surface of the mercury salt, in a solid frill 

 around the zinc. The copper wire in this cell became entirely 

 detached, partly because of the expansion upward of the 

 marine glue, which brought a severe strain upon the wire. 



The local action then 'increases zinc sulphate in the cell at 

 the expense of the mercury sulphate and amalgamates the zinc 

 rod. I have become convinced by some experiments extend- 

 ing over several weeks that this substitution process goes on 



