THE ALUMINUM ELECTROLYTIC CONDENSER 57 



But a limit is reached beyond which increases in specific resistance 

 produce objectionable additions to the electrical impedance of the 

 condensers, because of the high internal resistance set up in the cells. 



In a condenser, as shown in Fig. 6 at room temperatures of about 

 25° C, a fluid having a specific resistance of 75 ohms per cc. will 

 last from six months to a year without need for renewal because of 

 the precipitation of aluminum hydroxide. 



With fluid of 150 ohms per cc. the period of useful life is from 

 one to three years, and with a 300-ohm solution is upward of five 

 years, possibly never requiring renewal within the useful life of the 

 cell. The rate at which the precipitate forms in a given solution is 

 greatly accelerated at elevated temperatures and at 40° C, for example, 

 the solution remains free from a white precipitate only about one 

 third as long as at 25° C. 



With respect to the efi^ect of the acidity or alkalinity of the solution 

 on the operation of aluminum condensers the difference is not readily 

 distinguishable. Films can be formed and cells can be operated both 

 in acid and alkaline electrolytes, and the electrical characteristics, 

 except for resistance effects due to different solution conductivities, 

 are essentially the same in both kinds of electrolyte. Somewhat 

 better results, with respect to corrosion of electrodes have been ob- 

 tained, however, with alkaline solutions, particularly under unfavorable 

 operating conditions. 



The Relation between Voltage of Film Formation and 

 Operating Voltage 



Electrolytic condensers, used on circuits associated with storage 

 batteries, must be capable of operating at potentials throughout the 

 range of voltage variations due to charging and discharging the 

 batteries. Both the 24-volt and the 48-volt type condensers described 

 can be used in circuits up to 140 per cent of their normal voltage, 

 provision for this variation being made by the initial formation of a 

 film to a potential somewhat above the maximum operating value. 



After a condenser is connected in service the excess thickness of 

 film is removed slowly by the chemical action of the electrolyte, 

 because the film on the anode is maintained only at a thickness 

 corresponding to the operating voltage. Ordinarily, therefore, the 

 capacities of these condensers increase from their initial values, and 

 stabilize at new values depending upon the maximum potential in 

 the cycle of operating-voltage variations. 



