50 BELL SYSTEM TECHNICAL JOURNAL 



of aluminum condensers that must be controlled if the cells are to 

 operate satisfactorily. 



The first concerns the chemical action of the electrolyte on the 

 electrodes and the film. This action, which is merely a matter of 

 the film dissolving and forming aluminum hydroxide in the solution, 

 takes place when the cell is off circuit as well as when potential is 

 impressed. With impressed potential, new film forms under the 

 influence of the leakage current to replace that which is dissolved, 

 but in time the fluid becomes saturated with aluminum hydroxide, 

 which may precipitate as a white jelly and adversely affect the life 

 of the condenser. 



The second consideration involves corrosion of the positive elec- 

 trodes. The susceptibility of aluminum to corrosion is well known, 

 and in the use of electrolytic condensers anodic corrosion is the most 

 damaging irregularity that can occur. 



Obviously then an electrolyte must be chosen that does not rapidly 

 dissolve the film, and the material for the electrodes as well as for 

 the electrolyte must be selected and prepared to prevent serious 

 corrosion of the "formed" aluminum plates. 



Commercial Applications and Designs of Electrolytic 



Condensers 



Reference has already been made to the use in telephone systems 

 of electrolytic condensers. The principal applications of this device 

 involve its use in low-pass electric wave-filters. These filters are 

 placed in the supply circuits associated with central office storage 

 batteries to eliminate noise-producing ripples and pulsations, intro- 

 duced by battery charging-apparatus and signaling equipment, from 

 the direct current furnished to telephone instruments. That is, the 

 filters are used to exclude hum and other disturbing noises from the 

 subscribers' circuits. 



In Fig. 6 is shown an electrolytic condenser of the type designed 

 for direct-current filter service. When prepared for operation on 

 24-volt d.c. circuits, the capacity of this cell is nominally 1,000 mf. 

 at 1,000 cycles, and for 48 volts is about 600 mf. at the same frequency. 

 The cell is 8 inches wide, 10.25 inches long and 14.25 inches high 

 (20 X 26 X 36 cm.). Completely assembled it weighs about 42 pounds 

 (19 kg.), including 22 pounds (10 kg.) of electrolyte. 



The container for the condenser is made of heat-resisting glass 

 which reduces possible breakage due to temperature variations. The 

 electrodes, both of aluminum, are rigid and are bolted to a porcelain 

 cover to keep them in proper space relation. Two supporting bolts. 



