376 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



fa» 



TO CELL 



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TO GALVANOMETER 



TO BATTERY 



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minium anode at voltages below two volts are of the same order as 

 polarization capacities found for other metals in electrolytic cells. 

 They are, however, smaller than most of these, the maximum value 

 observed for aluminium being about 8 microfarads per square centi- 

 meter, while other metals often show several times this capacity. 



An examination of the various measurements we have made on dif- 

 ferent aluminium anodes shows a remarkable agreement in properties. 



It is possible to reproduce a 

 capacity with different samples 

 of aluminium, with electrodes 

 of different area, but which 

 have been formed at the same 

 voltage, with an accuracy ap- 

 parently as great as 2 per cent. 

 Other factors, such as tempera- 

 ture, electrolyte, time of charge, 

 discharge, and insulation, etc., 

 must of course be kept constant, 

 but when these conditions are 

 met, and notwithstanding the 

 complex nature of the film in- 

 volved, the capacity is a very 

 accurate function of the voltage 

 at which the plate has been 

 formed. 



2. Capacity below Forming 



Arrangement of switches for varying Voltage. — Figure 4 gives the 



short charge. Short insulation and long results of a set of measurements 



discharge. of capacity at voltages less than 



forming voltage, and this fam- 

 ily of curves gives an indication of the complexity of the active film. 

 The same times of charge, discharge, and insulation as were used in the 

 previous measurements were maintained in these. 



The plate was first formed at 6 volts, and measurements were taken 

 at 6, 4, and 2 volts. The results are plotted in the upper curve. 

 Formation was then continued, and completed at 10 volts, and the 

 results of measurements at 10, 6, and 2 volts are given in the second 

 curve. The other curves give similar results up to a forming voltage 

 of 41.6, measurements being made in each case at the forming voltage 

 and then at several lower voltages. It will be seen from this figure that 

 whatever the nature of the film may be, and whatever the mechanism 

 by which it acts, the capacity is greater at voltages lower than the 



Figure 5. 



