Electromotive Forces in the Voltaic Cell. 



71 



The method rests upon a remark of Helmholtz*, that a 

 quantity of mercury, if insulated, and arranged to form drops 

 quickly beneath the surface of a liquid, rapidly acquires the 

 potential of the liquid. Let T (fig. 1) be an insulated drop- 

 ping-funnel full of mercury, which drops inside the liquid F 

 through a fine point, and let M be a metal plunged into the 

 same liquid ; the electrometer E will indicate the true differ- 

 ence of potential between M and F. I have experimentally 

 convinced myself that, by careful regulation of the flow, one 

 can get the potential of the mercury to correspond with that 

 of the liquid within about -01 volt. 



Fig-. 1. 



Fig. 2. 



yMlii 



In order to measure the potential-difference of two liquids, 

 one uses two funnels (fig. 2), and connects each of the 

 two liquids F l and F 2 with a third vessel containing one of 

 them by means of a capillary siphon, so as to prevent any 

 mixture. I have to thank my friend Dr. Arrhenius for the 

 suggestion of the two funnels. I have found by this method 

 that pretty big potential-differences frequently occur between 

 liquids, going up to *8 or *9 volt with dilute ' solutions. The 

 E.M.F. of a Daniell cell appears to be about *8 volt between 

 zinc and zinc-sulphate, *3 volt between copper and copper- 

 sulphate ; while between zinc-sulphate and copper-sulphate 

 the difference seems practically zero. Nevertheless, since 

 copper salts have shown some hitherto unexplained phenomena, 

 I do not yet quite regard this result as indubitable. 



T am at present so immersed in literary work that I cannot 

 continue my investigations for several months ; but I should 

 be grateful if you will translate some of this and send it to the 

 Editors of the Philosophical Magazine. 



Believe me, &c, 



Dr. WlLH. OSTWALD, 

 Prof. Polytechnicum, Riga, Russia. 



* Monatsber. Berl. Ah. November 1881. 



