OVER-VOLTAGE PROTECTION 605 



universal protector against all kinds of interruptions. For 

 example, while it meets the usual, and most of the unusual, needs 

 in protection against disruptive potentials from lightning, an 

 arrester located in the station cannot, and is not expected to, pro- 

 tect an insulator out on the line from a lightning flash. Neither 

 is it designed to protect against surges of comparatively low 

 potential. 



The design is based on the characteristics of a cell consisting 

 of two aluminum plates, on which has been formed a film of hydrox- 

 ide of aluminum, immersed in a suitable electrolyte. This film is 

 formed on the aluminum plates by a series of chemical and electro- 

 chemical treatments at the factory. Up to a certain critical 

 voltage this hydroxide film has the property of insulating, or 

 rather opposing, the flow of current and is, therefore, closely 

 analogous to a counter electro-motive force. Up to this critical 

 voltage only a small leakage and charging current can flow, but, 

 during any rise above this voltage the current flow through the 

 cell is limited only by the actual resistance of the electrolyte, 

 which is very low. 



The action is comparable to that of the well-known safety 

 valve of a steam boiler by which the steam is confined until the 

 pressure rises to a given value, at which point the valve opens and 

 releases the excess pressure. This action of the aluminum cell is 

 also closely analogous to that of a storage battery on direct-cur- 

 rent. Up to about two volts per cell, the storage battery, when 

 charged, opposes an equal counter electro-motive force, shutting 

 off the flow of current; but for voltages above this value the cur- 

 rent is limited only by the internal resistance of the cell. This 

 characteristic makes the aluminum' cell ideal as a means of dis- 

 charging abnormal potentials or surges in electric circuits. It 

 practically prevents the flow of current at operating voltages, 

 but instantly short-circuits such abnormal portion of a potential 

 wave, or surge, as would be dangerous to the insulation of the 

 system. 



A volt-ampere characteristic curve of the aluminum cell on 

 alternating-current is shown in Fig. 387, and it should be noted 

 that the critical alternating-current voltage is between 335 and 

 360 volts. This curve gives the discharge rate only up to 5 am- 

 peres in order to better illustrate the normal and critical voltage 

 points. Above this value the discharge rate depends almost 



