56 Mr. J. Parnell on Secondary Currents 



solution slightly raises the figure of merit, but not to an extent 

 sufficient to counterbalance the disadvantage of a more rapid 

 efflorescence on the plates. A very few drops of a solution of 

 ammonia added to the sod.-carb. solution reduced its power by 

 75 per cent. A prolongation of the time of excitation caused 

 an augmentation of the figure of merit, which, when that time 

 is a variable, becomes a function of its reciprocal; and as cop- 

 per in sod. carb. attained very nearly its maximum force after 

 twenty seconds' excitation, whereas all other combinations re- 

 quired a much longer time, it is not thought necessary to burden 

 this paper with Tables of the results obtained. 



9. Although, as has been before stated (§ 4), by the mode of 

 manipulation adopted we practically eliminate all external resist- 

 ance (other than the resistance-coils) and their variations from 

 consideration as regards the galvanometer circuit, yet the varia- 

 tion of the internal resistance of the Daniell cell, which affected 

 the intensity of the exciting current, is of serious importance, and 

 was, no doubt, one cause of the variations which appeared in the 

 readings obtained on different days during the course of this 

 investigation, although on the same day the results were very 

 accordant. On this account the experiments were continued from 

 day to day till a large number of results had been obtained ; and 

 from these it appears that the relative powers of the metals and 

 solutions described in the Tables to this paper were always 

 maintained. 



10. There is another point to which attention was directed, 

 which was^ this — that the secondary couples did not lose their 

 power with the same rapidity, either during discharge or sponta- 

 neously; and as in the practical application of secondary bat- 

 teries, as well as in their scientific aspect, the persistence of the 

 current is of importance, experiments were undertaken to deter- 

 mine the rate of decadence of the powers of certain couples; and 

 some of the results are recorded in Table IV. : — 





Table IV. 





Seconds' 



Cu Cu 



Pt 



rest. 



sod. carb. pot. hyd. 



dil. sulph. ac. 



20 . 



. . 12 5 



30 



40 . 



. . 17 8-5 



43 



In these experiments, after the Daniell cell had been cut off 

 from the secondary couple, an interval of twenty or forty seconds 

 was allowed to elapse before contact was made with the galvano- 

 meter, and in the above Table IV. we have the percentages of 

 loss sustained by the secondary couples during those intervals. 

 We see that copper in a solution of hydrate of potassium lost 

 only 8*5 per cent, of its force during forty seconds, whereas pla- 

 tinum in dil. sulph. ac. lost in the same time 43 per cent. 



