1887.] On the Development of Feeble Currents. 269 



condition of equilibrium, different constant values are usually ob- 

 tained for the E.M.Fs. generated by opposing to the oxidisable metal 

 first one and then the other of any given pair of aeration plates, the 

 currents generated being rendered throughout of too small density 

 for "running down" to take place during the observations by inter- 

 posing a large resistance in the circuit. If when this state of con- 

 stancy has been attained the two aeration plates be opposed to each 

 other with a considerable resistance in circuit, a current passes from 

 the one giving the higher value when opposed to the oxidisable plate 

 through the external circuit to the other. This current at first is of 

 such magnitude as to correspond exactly with the E.M.F. due to the 

 difference between the E.M.Fs. exhibited when the two plates respec- 

 tively are opposed to the oxidisable metal, but after some time it 

 gradually diminishes ; even after several days, or even weeks, how- 

 ever, it is usually still measurable. If a miniature silver volta- 

 meter be included in the circuit, in many cases an appreciable amount 

 of crystalline silver is found to be slowly deposited on the negative 

 electrode of the voltameter, which may conveniently be a thin gold 

 wire immersed to a depth of a few millimetres in silver nitrate solu- 

 tion, a silver plate or wire forming the positive electrode. Thus, for 

 example, in various experiments the following figures were obtained, 

 the aeration plates being arranged on the surface of dilute sulphuric 

 acid : — 



Nature of aeration plates. 



Time in 

 days. 



Silver deposited 

 in voltameter in 

 milligrams. 



Average current 

 in microamperes 

 during the 

 period. 







1. Spongy platinum and smooth. 



14 



10-5 



7-7 



platinum foil 









2. Ditto ditto 



18 



1-5 



0-8 ' 



3. Spongy platinum and smooth 



24 



6 5 



2-8 



gold plate 









4. Spongy gold and smooth gold 



14 



10 



0-7 



gold plate 









5. Spongy gold and platinum 



24 



1-25 



0-5 



foil 









Analogous results were obtained in various other cases with dif- 

 ferent electrolytic fluids, e.g., spongy silver and smooth silver sheet 

 with caustic soda solution. In every case the action was greatest at first 

 and gradually diminished, but never became absolutely nil. The 

 larger the surface of the aeration plates, as might be expected, the 

 greater was the average current ; thus in experiment No. 1 above, 

 plates exposing about 20 sq.c. surface (one side, superficial measure- 



x 2 



