ana a new Secondary Battery. 57 



11. To test the permanency of the secondary current during 

 discharge, the resistance was reduced to 700 B.A. units, employ- 

 ing the 99 per cent, shunt. In the following Table V. we have 

 the figures of merit of copper in sod. carb. and platinum in dil. 

 sulph. ac. with 7000 units and 9 per cent, shunt, and 700 units 

 and 99 per cent, shunt respectively : — 



Table V. 

 Resistance .... 7000. 700. 



Copper .... 853 821 



Platinum. ... 772 570 



From this we see that, on diminishing the resistance of the 

 secondary circuit by more than 6000 B.A. units, the platinum 

 couple lost its force during the time (about three seconds) of the 

 swing of the galvanometer-needles much more rapidly than the 

 copper couple. 



12. Copper* plates were tried in a solution of the impure car- 

 bonate of sodium known in commerce as "soda;" and not only 

 did they rise more quickly to their full power, but the maximum 

 was far greater than that with the pure substance, giving a figure 

 of merit 920 — a value so high that, if it represented even the 

 whole of the electromotive force generated in the secondary 

 couple^ we might be inclined to think that further experiments 

 in this direction could lead to no better result ; but a portion of 

 that force was lost during the swing of the galvanometer-needles. 

 It will be seen further on that, when working on a larger scale 

 and with a more accurate method of measuring the electromotive 

 force, we can obtain a figure of merit higher even than that just 

 mentioned. 



13. The force developed with the copper-" soda n couple for 

 periods of excitation less than twenty seconds is shown in 

 Table VI., where the power of the couple after twenty seconds 

 is represented by 100 : — 



Table VI. 



Period of excitation, Force of 



in seconds. couple. 



2 75 



4 83 



6 88 



8 91 



10 94 



16 98 



20 . 100 



so that after ten seconds of excitation the force attained is within 

 about 6 per cent, of the maximum. 



* In this, as in all other alkaline solutions, copper was found to be far 

 superior to every other metal with which experiments were made. 



