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BOTANICAL GAZETTE 



calomel electrode as well as the exceedingly small contact potential 

 between the KC1 and the solution in A) was made by the Poggendorf 

 compensation method, with the apparatus so arranged that the 

 amount of current taken from the cell was reduced to a minimum. 



The calomel electrode was connected with one end of a slide wire 

 bridge (FG) and the platinum electrode was connected, through a 

 zero instrument H with the sliding contact. A Lippman capil- 

 lary electrometer (since it takes no current) was found more 

 satisfactory as a zero instrument than a galvanometer, which is 

 usually employed. To have the electrometer at zero at the begin- 

 ning of a reading, it was kept short-circuited through itself by 

 properly shifting the key /. 



A potential was maintained along the wire FG by means of the 

 battery K (3 dry cells in parallel). The variable resistance LM 

 permitted this to be regulated to suit the requirements. By con- 

 necting a voltmeter V between the negative end of the bridge and 

 the sliding contact, it is possible to read off the potential taken 

 from the battery circuit at any time. 4 



In taking a reading, the battery circuit through the bridge was 

 closed by the key N, and the circuit to be measured was immediately 

 closed by the key The sliding contact was then adjusted so that 

 the potential from the battery circuit was just sufficient to balance 

 that produced in the reaction beaker A . When this condition was 

 reached no current flowed and the electrometer was in equilibrium. 

 The reading of the voltmeter then gave the potential of the cell. 



The potential produced at the platinum electrode could then 

 be determined by subtracting the positive single potential of the 

 calomel electrode, usually referred to as 0.56 volt, from the total 

 potential observed. For comparative purposes, however, since the 

 potential of the calomel electrode was constant, the total potential 

 of cell may often be used. 



The curve of the oxidation of oxalic acid by acidified potassium 

 permanganate, shown in fig. 2, illustrates the way in which a reac- 

 tion may be followed by this method. On placing a platinized 



