284 



Scientific Proceedings, Royal Dublin Society. 



current through the voltmeter produces a small polarisation which has a 



slight disturbing effect, even if the voltmeter key is only depressed when 



taking observations. In spite of these defects, the results were sufficiently 



accurate, in all cases, to be cast into the form of equation (2). We shall 



merely reproduce here the observations for an exceptionally large polarisation 



obtained for a cell which had been in use for some time. The recovery 



curve was also very slow. In consequence it was easy to obtain accurate 



readings with the voltmeter. As will be seen by reference to fig. 2, the linear 



relation required by equation (2) is well satisfied. We find the linear graph 



gives 



(1-32 - F)- 1 = 0-694 t min + 1-38. 



The manner in which this equation represents the observations may be seen 

 from Table I. 



Table I. 

 Recovery of a Leclanclie Cell polarised by 0'32 amp. for 5 minutes. 



The agreement is generally close, except at the beginning, where the 

 reading of the voltmeter cannot record accurately the value of the E.M.F. on 

 opening the circuit, and towards the end where the slower rise has begun to 

 exert its influence. 



Another example may be taken from observations for polarisation with 

 small currents made by a modified potentiometer method. A potentiometer 

 was arranged to measure the E.M.F. of the cell, using as a galvanometer a 

 Paul unipivot which had a sensibility of one division per microampere. 

 Small deviations from the null-point were measured by the deflection of the 

 galvanometer, which was shunted off a rheostat so as to give readings over the 

 full length of the scale. A Pohl commutator was arranged as a three-way 

 key so that throwing it over opened the cell circuit and connected the cell to 

 the potentiometer. The following device was used to set the potentiometer 

 to the voltage of the cell immediately on opening the circuit. The cell circuit 

 was closed until the polarisation had become nearly steady. The key was 

 then rocked backwards and forwards so that the cell was alternately either on 

 closed circuit or connected to the potentiometer. The null-point so obtained 

 is the voltage of the cell immediately on opening the cell circuit, for the 



