370 PROCEEDINGS OF THE AMERICAN ACADEMY. 



in which 6 may have any value from zero to 90°. — f - — J is the lag 



of p behind E, and 6 depends on the nature of the electrodes and the 

 electrolyte. 



This theory has been built up on the basis of Nernst's theory for 

 the single electromotive force of an electrode in a solution containing 

 its ion, and the theory fits the majority of cases very closely indeed. 

 For most metals the maximum of polarization lies below three volts. 

 This means that if we raise the electromotive force applied about the 

 cell beyond three volts, the polarization no longer increases. This is 

 true whether the electrode is a gaseous one, a reversible one (a metal 

 in contact with a solution of its own ion), or any other combination of 

 metal and electrolyte. In all of these cases there enters into the 

 equation of the polarization electromotive force the ratio of concentra- 

 tions in the ordinary Nernst form 



E = —r= In 77. 



It is then a familiar fact that the polarization electromotive force 

 does not rise above three volts in any ordinary electrolytic cell. It is 

 possible to raise the voltage of a cell having an aluminium anode at 

 least as high as 500 volts, and it is possible to raise the voltage about 

 cells having anodes of other metals, tantalum for example, to 1200 

 or even 1500 volts without reaching a point corresponding to the 

 maximum of polarization as found for ordinary metals. If an alu- 

 minium anode has been properly "formed," that is, exposed to an 

 electromotive force which is slowly increased step by step, the cell 

 offers a remarkably complete barrier to the passage of a current. A 

 small residual current flows through the cell under these circumstances, 

 but this falls to a few milliamperes per square centimeter of electrode 

 surface even when the applied electromotive force is measured in hun- 

 dreds of volts. It seems quite evident that the process which takes 

 place here is not polarization in the ordinary sense of the word. The 

 substitution in the Nernst equation of the value for the "counter elec- 

 tromotive force " of a cell containing an aluminium anode leads to 

 what appear to be absurd values for the ratio of the concentrations oi 

 the ion at the electrode and in the electrolyte. 



IV. Theories of the Aluminium Anode. 



The special characteristic of aluminium and a few other metals ap- 

 pears to be a film which forms on the metal when it is used as anode, 



