877 
Al-electrode in 1/2 N Al2(SO4)3 
i/o - anode 
0.8 | + 2.56 
| 0.53 + 3.48 
| 0.46 + 3.84 
0.36 + 4.13 
It has been tried to account for this phenomenon by assuming 
the formation of an AI,O,-layer with great resistance, which sup- 
position is, however, hardly tenable, for when the above-mentioned 
phenomenon presents itself, the aluminium-anode is perfectly bright. 
Besides when the current is reversed, the resistance has entirely 
disappeared. 
When the tension is increased, there is actually formed a coat of 
Al,O,, Al(OH), or of a basic salt. Then the density of the current 
is practically reduced to zero, but when the current is reversed, the 
potential of the aluminium-electrode is considerably smaller. 
When the anode potential is carried up very high, e.g. to 200— 
500 V., the potential is reduced to from '/,, to '/,, on reversal of 
the current. 
This property, the so-called valve-action, is used to transform an 
alternating current into a continuous one. With high current-densities 
the electric valve-action stops under ordinary circumstances owing 
to rise of temperature. 
Fiscuer*), therefore, used as anode an aluminium tube, through 
which water flowed, and in this way he succeeded in getting coats 
of oxide of a thickness ot some tenths of millimeters. 
The most extensive researches on the valve-action of aluminium 
have been performed by Scnurze*). He assumes, that every newly- 
1) Zeitschr. f. phys. Chem. 48, 177. 1904. 
*) Ann. der Phys. 21, 929. 1906. 
Ply, voe 548! 1907: 
» ¥ - 23, 226. 1907. 
- ‘ » 24, 43, 1907. 
» 5 = 2010008; 
. , . 28, 787. 1909, 
See 34, 667, 1911. 
s = . 41, 598. 1913. 
Zeitschr. f. Elektrochem, 20, 307. 1914. 
. " 9 20, 592. 1914 
