190 ANNUAL EECOKD OF SCIENCE AND INDUSTRY. 



influence of the rotation of the magnet upon the intensity 

 of the permanent current is scarcely less obscure than that 

 of the unilateral conductivity; and the explanation of the 

 one phenomenon involves the apparent contradiction of the 

 othei\ Bull Hebcl, 1875, 297. 



SINGULAR PROPERTY OF ALUMINUM ELECTRODES. 



A singular property of aluminum has been noticed by 

 Ducretet. A voltameter, whose electrodes are respectively 

 aluminum and platinum, allows an electric current to pass, 

 or prevents it, according as the electrodes are respectively 

 positive or negative. If the positive current passes from 

 the platinum to the aluminum, no unusual resistance is ex- 

 perienced. If, on the other hand, a positive current passes 

 from the aluminum to the platinum, the current is nearly ar- 

 rested, and the needle of the galvanometer marks in the first 

 case, for instance, 22, and in the second case 2 only. The 

 explanation of this seems to be that in the former, or favora- 

 ble case, the current disengages oxygen at the platinum and 

 hydrogen at the aluminum pole. But in the opposite case 

 the oxygen is produced at the aluminum pole, and forms a 

 layer of alumina (or the oxide of aluminum), to the presence 

 of which the arrest of the current must be attributed. If 

 the poles are placed in hydrochloric acid, the phenomenon 

 no longer takes place, and similarly does it not follow if 

 we employ any alkaline liquid. In opposition to this expla- 

 nation, however, it must be granted that the microscopic 

 examination of the aluminum electrode does not reveal any 

 apparent change in its appearance, no matter in which way 

 the current flows. Whatever may be the explanation, the 

 fact remains, and is certainly a very striking one. Of 

 course, if both electrodes are formed of aluminum, the cur- 

 rent Avill not flow in either direction. Ducretet proposes to 

 apply this interesting property of aluminum in the construc- 

 tion of a telegraphic apparatus, which he calls a rheotome of 

 constant direction. 13 _Z?, III., 218. 



ON THE ELECTRIC DISCHARGE. 



The electrical sparks of a peculiar nature that have been 

 called "feeble sparks" by Riess, by whom they were first 

 discovered, are distinguishable from the ordinary bright 



