OVER-VOLTAGE PROTECTION 607 



characteristic of the cell of great importance. The thin insulating 

 film of aluminum hydroxide between the conducting aluminum 

 and the conducting electrolyte acts as a dielectric, and the cell, 

 therefore, is an electrostatic condenser. Due to this capacity, 

 however, aluminum arresters cannot be connected permanently 

 to the circuits and horn gaps are, therefore, inserted in series with 

 the connections. 



Another characteristic of the aluminum cell is the dissolution 

 of a part of the film when the plates stand in the electrolyte and 

 the cell is disconnected from the circuit. The film is composed of 

 two parts; one part is hard and insoluble, and apparently acts as 

 a skeleton to hold the more soluble part. The action of the cell 

 seems to indicate that the soluble part of the film is composed of 

 gases in a liquid form. When a cell which has stood for some time 

 disconnected is reconnected to the circuit, there is a momentary 

 rush of current which re-forms the part of the film which has dis- 

 solved. This current rush will have increasing values as the inter- 

 vals of rest of the cell are made greater. If the cell has stood dis- 

 connected from the circuit for some time, especially in a warm 

 climate, there is a possibility that the initial current rush will be 

 sufficient to open the circuit breakers or oil switches. This cur- 

 rent rush also raised the temperature of the cell, and if this tem- 

 perature rise is great it is objectionable. When the cells do not 

 stand for more than a day, however, the film dissolution and initial 

 current rush are negligible. Suitable means, as later described, 

 are provided with the arresters for throwing them directly on the 

 line and charging them by a very simple operation, and thus the 

 film may be always kept in good condition. 



The aluminum lightning arresters for alternating-current cir- 

 cuits from 1000 to 155,000 volts consist essentially of inverted 

 aluminum cones arranged in stacks and insulated from one another 

 (Fig. 388). An electrolyte partially fills the space between adjacent 

 cones, so forming aluminum cells connected in series. The stacks 

 of cones with the electrolyte between them are then immersed in 

 a tank of oil. The electrolyte being heavier than the oil remains 

 between the aluminum cones. Between the stack of cones and 

 the steel tank, tubes of insulating material are placed. These 

 improve the circulation of the oil and increase the insulation 

 between the live parts. The oil improves the insulation between 

 cones, prevents evaporation of the solution and, due to its heat- 



