The Aluminum Electrolytic Condenser

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 S I N C E the discovery about 75 years ago 2 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 resistance per 1,000 sq. cm. of film on an aluminum anode in an ammonium borate electrolyte 3 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.