THE ALUMINUM ELECTROLYTIC CONDENSER 45 



proportional to the potential at which the film is formed, indicating 

 that the thickness of the dielectric is directly proportional to the 

 voltage of formation. 



Effect of Impressed Voltage on Capacity 



When an electrolytic cell with a "formed" anode has impressed on 

 its terminals a voltage greater than the formation voltage, the film 

 must build up to the new potential before the electrical characteristics 

 of the cell become stable. At this higher voltage the capacity of the 

 cell will be reduced to correspond to the increased potential. Wher.^ 

 large plate areas are involved the direct application of a potential 

 above the formation voltage results in a heavy flow of current, which 

 may overheat and damage the cell if not properly limited. 



If a voltage is impressed on a condenser lower than the potential 

 applied during the formation of the film, the cell will operate satis- 

 factorily, but the capacity will not be immediately affected and will 

 correspond to the potential at which the film was originally formed. 

 This is illustrated in Table I, which shows the capacities of an electro- 

 lytic cell measured with applied voltages of different values below the 

 voltage of formation. 



However, if a condenser operates for a long time at a reduced 

 voltage the excess film will be removed slowly by the chemical action 

 of the electrolyte, and the capacity will increase gradually to a value 

 depending upon the operating voltage. The rate of change of capacity 

 under these conditions is affected by the temperature at which the 

 cell operates, and by the conductivity of the electrolyte. As is 

 illustrated by the curves shown in Figs. 3 and 4 the change becomes 

 more rapid when these factors are increased. 



TABLE I 



Applied Potential Capacity Readings 



Volts — D.C. Microfarads 



49.2 1008 



43.0 1002 



36.7 1008 



30.4 1013 



24.3 1013 



18.3 1013 



Series Connected Condensers for Direct and Alternating 

 Current Service 



In the discussion of the current- voltage relations in Fig. 1, it was 

 noted that an aluminum cell with one electrode of non-film-forming 

 metal conducts current freely when the aluminum is cathode. Accord- 

 ingly this type of cell is capable of holding a charge of electricity and 



