.262 Prof. Potter. Electrical Effects accompanying the [July 14, 



to avoid the necessity of long platinum wires a short length of this metal 

 was soldered with silver to a copper wire. For the purpose of insulation 

 the wire was enclosed in a glass tube, the extremities of which were then 

 fused round it, leaving a projection of platinum wire at one end and copper 

 wire at the other, the former only being brought in contact with the nutrient 

 fluid. Other forms of electrodes will be mentioned later. The leads from 

 the electrodes were connected with a standard condenser of one micro-farad 

 and the condenser discharged through a galvanometer by means of a Morse 

 key. This arrangement was adopted, on the suggestion of Mr. H. Morris 

 Airey, to eliminate the resistance of the circuit. A Clark's cell discharged 

 in the same manner through the galvanometer served to show the number 

 of deflections upon the galvanometric scale, which corresponded to the 

 standard voltage. 



The electrodes were carefully tested, and it was quite certainly ascertained 

 that no E.M.F. was developed at the junction of the platinum and copper. 

 The cell when charged was also thoroughly tested to determine whether any 

 E.M.F. produced could be due to differences in temperature between the 

 media separated by the porous cylinder, to osmotic effects, or to evaporation. 

 It might be supposed that a possible difference of temperature between the 

 junctions of copper and platinum might give rise to a thermo-electric effect, 

 but this point was decided by heating the outer fluid several degrees above 

 that of the inner, and under these conditions no E.M.F. was developed. 

 Again, many trials were made, by varying the concentration of the solutions 

 outside and inside the porous cylinder, to prove whether the E.M.F. could be 

 produced by reason of osmotic effect, but this was found not to be the case. 

 Evaporation from either the inner or outer fluids was also effectually 

 prevented, and thus any E.M.F. which might be developed could not be 

 attributed to evaporation currents. 



As oxygen is not liberated in any of the reactions considered in this 

 investigation, no electric effects could be produced by oxidation of the 

 platinum electrodes. 



Platinum, it is well known, often contains a local charge, and a difference of 

 potential might thus be registered upon the introduction of the two electrodes 

 into the solutions. Experience showed that this local charge occurred, but 

 from repeated tests with control cells it was proved to be constant during 

 the limits of an experiment, and allowance for this effect could be made. 

 This relative charge upon the platinum wire, however, is altered by friction, 

 and it is essential to prevent any shaking or other disturbance of the 

 electrodes during an experiment. The leads from the electrodes were there- 

 fore, fixed to a support, so that no disturbance of the electrode was possible 



