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REPORT — 1901. 



Liquid Electrolytes, and cousoquently they establish the analogy between con- 

 duction of electricity by salt solutions and that by salt vapours. 



8. Preliminary Note on the Theory of the Lippnxann Electrometer and 

 related Phenomena. By F. G. Cottrell. 



In the paper it is pointed out that in the determinations of single potential 

 differences between metals and solutions of their salts by means of either the 

 capillary electrometer or dropping electrodes the assumption has up to the 

 present been made that the presence of a large amount of ' indifferent ' and gooc 

 conducting electrolyte in uniform concentration throughout effectually prevents 

 the differences of concentration of the metallic ions within the solution from 

 producing any measurable electromotive forces. This is shown to be the case 

 only— 



(1) When the total quantity of depolarising agent (usually a mercury salt) in 

 the dilute portion of the solution (layer next the mercury in the capillary) is large 

 in comparison to that used up at the electrode during the measurements ; or 

 (2) when the depolarising agent can diffuse from the concentrated to the dilute 

 portion in a practically undissociated state. 



In none of the forms of capillary electrometer or dropping electrode as yet 

 employed for quantitative measurement is the first of these alternatives satisfied 

 for such electrolytes as dilute sulphuric or hydrochloric acids or potassium 

 chloride. It is, however, for those (such as certain strengths of alkaline sulphide 

 or cyanide solutions) in which the unpolarised mercury is already at its maximum 

 surface tension. The second alternative is also not satisfied by solutions of the 

 strong mineral acids or their salts, but probably is by cyanides, and to some 



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