44 BELL SYSTEM TECHNICAL JOURNAL 



Upon reduction of the voltage, however, the insulating properties 

 of the film are restored and the current decreases with decreasing 

 potential in substantially the same relation to voltage as before. 

 The sparking over the surface will be observed to cease at about 

 the same potential at which it began, the glow will disappear and the 

 low leakage-current values will be obtained when the voltage is reduced 

 sufficiently. 



Upon reversal of potential on the aluminum electrode, however, 

 there is a much larger flow of current, the value of which is limited by a 

 counter voltage of several volts, and by the low internal resistance of 

 the cell with negative potential applied. Typical current-voltage 

 relations for an aluminum cell are shown graphically in Fig. 2. These 

 relations correspond to the curve in Fig. 1, showing the variation 

 of resistance with potential of a "filmed" aluminum electrode in 

 ammonium borate electrolyte. 



Capacity of Aluminum Films 



Like the ordinary paper or mica static condenser, the electrolytic 

 condenser consists of two conducting surfaces separated by an insu- 

 lator. The high-resistance film constitutes the insulator in the 

 electrolytic cell, and the electrolyte on one side of the film and the 

 metal of the film-bearing electrode on the other provide the two 

 conducting surfaces. The cathode in this type of cell merely provides 

 a means for making electrical contact with the electrolyte. 



When a film is formed upon a smooth polished aluminum surface 

 the coating is transparent. If observed under favorable illuminating 

 conditions the "filmed" surface is seen to be colored and may be 

 either green, yellow, red or blue, depending upon the thickness of 

 the film. This is attributed to light interference and indicates that 

 the thickness of the film is in the order of the length of light waves. 

 The actual thicknesses of films on aluminum have been determined to 

 be from 0.001 to 0.00001 mm.,^ depending upon conditions of for- 

 mation. 



Because of this extreme thinness of the dielectric and its high 

 insulation resistance when positive potential is applied, unusually 

 large capacities per unit area of surface can be obtained. The capacity 

 of a film formed to 30 volts on aluminum is about 0.18 microfarad 

 per sq. cm. of dielectric surface, or about 1,000 times that of paper 

 condensers. The capacity per unit area is approximately inversely 



^Zimmerman, Trans. Am. Elecirochem. Soc, 7, 309 (1905); Sutton and Willstrop, 

 Engineering, 124, 442 (1927); Slepian, Trans. Am. Electrochem. Soc, September, 

 1927, to be printed in Vol. 54 of the Transactions. 



