54 BELL SYSTEM TECHNICAL JOURNAL 



This is illustrated by the performance of groups of condensers having 

 anodes of different compositions as represented in Fig. 9. This 

 curve shows the percentage of the cells in each group affected by 

 corrosion in relation to the amount of silicon in the anode aluminum. 

 It will be noted that the group of condensers with anodes of the purest 

 metal (that is, least silicon) gave the least satisfactory results. 



The Negative Electrode 



The negative electrode consists of five rectangular flat plates, 

 having a combined useful surface area about 35 per cent of the total 

 positive surface. The negative plates or cathodes are of aluminum, 

 but they do not have a film formed on them because their sole function 

 is to provide contact with the condenser fluid. In an ammonium 

 borate electrolyte, such as is used in these condensers, there are a 

 number of other materials, including tin and carbon which can be 

 used for the negative electrodes. 



However, aluminum was chosen because it is light, relatively 

 strong, easily worked and mounted, and can be cleaned by the same 

 process used to clean the positive plate metal. A question might 

 properly be raised as to the use of an aluminum negative electrode, 

 particularly one having less area than the positive, because the 

 formation of a film on the negative electrode will result in a reduction 

 of the electrostatic capacity of the cell. 



In normal operation with aluminum negatives there is a tendency 

 for a film to form, even though the condenser is operated on direct- 

 current circuits, because the negative electrode is an anode during 

 the interval that the condenser discharges. However, this dis- 

 advantage with respect to the use of aluminum cathodes is overcome 

 by making these plates of metal, which is really a rich aluminum 

 alloy, containing enough other substances such as silicon to impede 

 the formation of a film on its surface. 



With material containing less than 99 per cent aluminum it is 

 possible to have as much as 3.5 amperes a.c. in the condenser circuit 

 or an alternating-current density of about 1 milliamp. per sq. cm. of 

 negative plate surface without forming sufficient film on the negative 

 plates to affect the capacity of the cell. 



Prevention of Contamination in Manufacture 

 AND Installation 



The initial formation of the film on the positive electrodes is carried 

 out by chemical cleaning and electrochemical formation processes, in 

 which the purity of the materials used as well as the composition of 



