3G8 PROCEEDINGS OF THE AMERICAN ACADEMY. 



in the rectification of an alternating current. Below a certain critical 

 voltage, which is a function of the electrolyte and the temperature, the 

 film which forms on an aluminium plate is a more or less efficient valve, 

 which permits of the passage of an electric current in one direction 

 and not in the other. The same facts determine the application of an 

 aluminium plate as a condenser. 



II. Historical. 



Wheatstone (74) appears to have been the first to notice the anom- 

 alous behavior of an aluminium anode, and he mentions it merely in 

 connection with an investigation on the position of various metals in 

 the voltaic series. Soon afterward Buff (10) noticed the remarkable 

 fact that a battery of nine Bunsen cells was insufficient to cause the flow 

 of an appreciable current through a voltaic cell in which aluminium was 

 anode. In 1869 Tait (72), using more delicate apparatus, measured 

 the polarization produced at anodes of various metals and found a very 

 high polarization electromotive force to be characteristic of aluminium. 

 During the twenty years following this date a very great number of 

 measurements on galvanic polarization and polarization capacity of 

 electrodes were made all over the world, and the anomalous behavior 

 of an aluminium anode was the subject of frequent notice. The first 

 suggestion that this property might be made use of in the rectification 

 of an alternating current appears to have been offered by Ducretet (22), 

 and occasional suggestions of the possibility of using aluminium plates 

 immersed in a proper electrolyte as a substitute for a static condenser are 

 to be found in these earlier papers. The first actual measurement of the 

 apparent capacity of such a cell is perhaps that of Streintz (69), who 

 showed that a formed aluminium anode can be used in this way, meas- 

 uring the capacity of the plate up to 28.8 volts. He assumed, as 

 many others have done, that an aluminium anode acts like a nearly 

 perfect condenser, and that a short time of insulation between charge 

 and discharge introduces no error into the measurement and may 

 therefore be neglected. Oberbeck (52) calculates the capacity per 

 square centimeter of anode surface, and from this value, assuming a di- 

 electric constant, he also calculates the thickness of the active insulat- 

 ing film. Application of aluminium plates immersed in an electrolyte 

 as a substitute for an ordinary condenser for practical purposes was 

 suggested by Haagn (34) in 1897. Pollak (55) had already tested 

 the aluminium rectifier practically, and Graetz (29), working quite in- 

 dependently, also showed the possibility of applying the properties of 

 an aluminium anode in the commercial recticfiation of an alternating 

 current. This was in 1897, and by far the greater part of the scien- 



