1888.] upon the Potential of Voltaic Couples. 36ft 



but with a Thomson's reflecting one of 3040 ohms resistance, it was 

 between 775,000 and 930,000. 



The effects obtained with zinc as a positive metal have already 

 been given (' Roy. Soc. Proc.,' May 31, 1888). With that metal and 

 the astatic galvanometer the minimum proportion of acid required to 

 change the potential lay between 1 part in 9,300,000 and 9,388,185 

 parts of water ; but with the reflecting one it lay between 1 in 15,500,000 

 and 23,250,000. 



Notwithstanding the electromotive force of magnesium is so much 

 larger than that of zinc in the very dilute acid, the minimum propor- 

 tion of the acid required to destroy the balance was very much 

 smaller with zinc than with magnesium, and the increase of electro- 

 motive force was more rapid with zinc than with magnesium. The 

 minimum proportion of acid required to change the potential with 

 magnesium (' Roy. Soc. Proc.,' May 26, 1888), or with zinc, was 

 nearly the same, whether the couple was balanced by a precisely 

 similar one or by the thermo-electric pile. The order of variation of 

 electromotive force by change of strength of the liquid was very 

 similar with zinc to what it was with magnesium, and the curves 

 generated by plotting the results were much alike. 



Table II.— Cd + Pt + HC1 in 465 grains of Water at 17'5° C. 



Grains. 



Volts. 



Grains. 



Volts. 



Grrains. 



Volts. 



0-15 



0-9494 



0-07575 



0-9108 



0-0009 



0-7678 



0-13563 



0-9108 



-06078 



9251 



0-00081 





12066 



» 



0-04581 



-9427 



-00073 



0-7478 



0-10569 



0-03084 





water 





'09072 





-01584 



0-9451 



















With the astatic galvanometer, the smallest proportion of acid 

 required to alter the balance was between 1 in 574,000 and 637,000 ; 

 but with the reflecting galvanometer it was between 1 in 1,162,500 

 and 1,550,000. The order of change, or curve of electromotive force 

 by variation of strength of liquid, was somewhat similar with cadmium 

 to what it was with zinc and magnesium. 



