The Aluminum Electrolytic Condenser ^ 



By H. O. SIEGMUND 



Synopsis: In this paper the anodic film-forming properties of aluminum 

 are discussed and the unique electrical qualities of film-coated aluminum 

 anodes are described. Special reference is made to an aluminum electrolytic 

 condenser of the type used in low pass electric wave-filters of direct-current 

 telephone power plant equipment. Electrical characteristics of condensers 

 are given and the manner is described in which the operation and life of the 

 units are influenced by variations in composition of the electrodes and the 

 electrolyte. 



Introduction 



SINCE the discovery about 75 years ago - of the unusual polarizing 

 effect of aluminum it has become well known that certain metals, 

 notably aluminum and tantalum, as anodes in a suitable electrolyte 

 become coated with a film having remarkable electrical properties. 

 Films formed in this manner are characterized by the influence of 

 impressed potential on their electrical resistance. 



A representative relationship between applied voltage and re- 

 sistance per 1,000 sq. cm. of film on an aluminum anode in an 

 ammonium borate electrolyte ^ is shown in Fig. 1. This resistance 

 characteristic imparts to the film the capability of conducting current 

 more freely in one direction than in the other; of breaking down as 

 an insulation between the metallic electrode and the solution when 

 voltages above a critical value are applied; and in combination with 

 the thinness of the film of holding a substantial charge of electricity 

 at potentials below the breakdown voltage. 



Each of these characteristics provides the principle around which a 

 distinctive class of electrical apparatus has been developed. The 

 electrolytic rectifier, widely used in small direct-current supply sets for 

 battery charging and radio purposes, employs the unidirectional con- 

 ducting characteristic. The aluminum electrolytic lightning arrester, 

 used extensively for protection of direct-current railway equipment, 

 depends for its operation upon the breakdown characteristic of the 

 film. And finally the aluminum electrolytic condenser, now being 



1 Presented before the American Electrochemical Society, at Bridgeport, Conn., 

 April 26, 1928. 



2 Wheatstone, Phil. Mag., 10, 143 (1855). 



^ The exact values of resistance are somewhat unstable, and depend on the time 

 between readings and whether successively increasing or decreasing values of potential 

 are applied. However, the general shape of the curve and the magnitude of the 

 values are representative. 



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