as a Standard of Electromotive Force. 131 



and all observations have confirmed the conclusions of Dr. 

 Alder Wright that the most uniform results are obtained from 

 copper plates freshly electrotyped over with a new clean 

 pinkish surface of virgin copper, when the plate is taken 

 straight out of the electrotyping bath and put into the cell 

 without any delay or touching except a slight rinse with 

 distilled water. The result of comparing a freshly electrotyped 

 rod with a rod scraped and glass-papered, was to show that 

 the scraped copper rod gave a higher electromotive force by 

 about 4 parts in 629, or 6 parts in 1000. When both rods 

 were freshly electrotyped in the same bath, and inserted in 

 the same cell, there was absolutely no difference between 

 them. Cells were compared with solutions of equal density 

 1/2 at 15° C. and amalgamated pure cast zinc, but different 

 copper rods, as follows : — 



E.M.F. 



Electrotyped copper rod . . 1*105 volt. 



Scraped copper rod . . . 1*111 „ 



The exact amount of difference varied slightly in oilier 

 experiments ; but with equally clean surfaces, the electrotyped 

 fresh surface has a lower electromotive force than the scraped. 

 The effect of amalgamating the copper rod is, like electro- 

 typing, to lower its value below that of the clean scraped, but 

 it is not so uniform. The following are values obtained from 

 equal copper rods in the same cell, one having an electro- 

 plated surface and the other being slightly amalgamated with 

 pure mercury : — 



Freshly electro-deposited Freshly amalgamated 

 copper rod. copper rod. 



1-103 1-104 



1-106 1-104 



1-105 1-104 



1-106 1-105 



The mean of the values of the electro-deposited copper 

 exceeds by about 1 in 1000 the mean of the amalgamated 

 copper ; but if the amalgamation is done thickly it causes 



paper in the Fhil. Trans, part ii. (1884 ) as 1*435 true volt at 15° (J. My 

 standard, called hereafter F 20 , is greater than this by 3 parts in 4940, or 

 is l*4oG volt at 15° C. ; and experiments have shown that its variation- 

 coefficient is nearly that found by Lord Rayleigh, viz. '082 per eent. per 

 degree diminution of E.M.F. 



Accordingly, the value of this standard Clark cell V 20 has been taken 

 as equal to 



1-436(1— -00082 P) volt at t° C, 



and all values given in the text are in terms of I 



K 2 



