88 Mr. I. B. Cooke on the Measurement of Chemical Affinity » 



comparably greater than the sum of all the other resistances of 

 any circuit of ordinary dimensions and conducting capacity into 

 which it could be introduced ; and as by its use the sources of 

 inconstancy could be eliminated from all such circuits, and their 

 resistances reduced to one conmion standard, the currents they 

 respectively developed would be obtained at the galvanometer in 

 direct proportion to their initial unweakened intensities ; or, in 

 other words, to the balance of the affinities exerted between the 

 elements present, before those affinities can in any degree be 

 satisfied by combination. 



If, while a cell was thus connected with the resistance tube 

 and galvanometer, the circuit was additionally completed by a 

 short thick wire joining the anode and cathode, the deflection of 

 the needle was of course destroyed ; and after a certain lapse of 

 time, if the wire was suddenly removed, the first swing of the 

 needle revealed the amount of diminution of current affinity 

 caused by the continued unobstructed action of the elements 

 during that space of time. 



In the case of a pair of zinc and copper in rain-water, the 

 diminution of force was found to vary witli the time of continu- 

 ance of the connexion, with the resistance of the circuit, and 

 with the excess in size of the zinc plate over that of the copper. 

 The current was never reduced to zero, but each individual cell 

 attained to a diff'erent minimum. 



On removal of the short connecting wire, the pair immediately 

 commenced slowly to recover their original force, which, how- 

 ever, could almost instantly be fully restored by taking out the 

 copper plate, and after waving it in the air, replacing it in the 

 cell. No such result was produced by a similar treatment of the 

 zinc plate, if efi'ected without disturbance of the copper, or of 

 the medium in which it was placed. 



If the copper plate was much larger than the zinc, and the 

 pair was placed in a stream of water, the maximum current was 

 maintained, even after a connexion by a thick wire of some 

 moments' duration. , 



If a single cell of Smee's batteiy was joined in series with a 

 pair of zinc and copper in water, the inconstancy of the latter 

 could be extended to the zero-point ; and indeed, as soon as 

 bubbles of hydrogen covered the surface of the copper plate, the 

 cui-rent became reversed, the copper being then slightly positive 

 to the zinc. 



-5 1 In considering the sources of the current in these simple cir- 

 (!uit8, it appears fair to assume that no affinity is exerted, when, 

 under favourable circumstances, no combination is efiected. 

 Thus the development of nascent hydrogen upon the copper plate 

 is favourable to the combination of the hydrogen and copper. 



