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XXXVI. The Clark Cell when producing a Current, 



To the Editors of the Philosophical Magazine. 



Gentlemen, 



IX the Philosophical Magazine for March 1895 you have 

 published a letter from Professor Threlfall criticising my 

 experiments on the Clark cell when producing a current. I 

 should like to reply to some points in it. 



Professor Threlfall quotes my definition of the quantity 

 which I have called the polarization of the cell. The method 

 of determining this quantity depended on measuring sepa- 

 rately the resistance of the contents of the cell treated as an 

 electrolyte and of the wire through which the cell was used to 

 maintain a current. Knowing the current, I calculated the 

 E.M.F. required to maintain it in an equivalent wholly 

 metallic circuit having a resistance equal to the sum of these 

 two values. The difference between this E.M.F. and that of 

 the cell at rest I tabulated as the polarization with varying 

 current-density. This point was considered very carefully by 

 me at the time, and it appeared that the resistance of the con- 

 tents of the cell could only be changed through unequal 

 migration of ions and electrolytic endosmose. Now the effect of 

 these two causes would be permanent when the cell was again 

 placed on open circuit ; and so if measurements taken before 

 and after the cell was used did not show any wide divergence, 

 it is legitimate to assume that whilst the current was flowing 

 the resistance would lie somewhere between these two values. 

 This is the reason which led me to adopt this method of 

 statement of results. 



The actual measurement of the resistance of the cell showed, 

 as I expected, very little change. The testing-current was 

 of the order of *007 ampere. 



The temperature of the 147 ohm wire through which the 

 current of approximately '01 ampere was maintained could 

 not have been far different from that of the oil-bath. A 

 current of this magnitude would only produce 12*6 gram 

 Centigrade thermal units in one hour; and as the wire 

 (20 grams) was openly wound " on an ebonite frame" it does 

 not seem likely that it would have been much hotter than the 

 oil. It would require a rise of 10° C. to produce a change of 

 *6 ohm. The case is not at all comparable with Mr. Griffiths', 

 where current up to an ampere was used. Besides, it must 

 be remembered that any possible error in the resistance of 

 this wire would have very little influence on the magnitude 

 of the quantity which I have called the polarization of the cell. 



2C2 



