Figs. 2,356 and 2,357.—Diagram showing condenser action of cylinders and pressure gradient for static stress.
The alloy contains a metal of low boiling point which prevents the reversal of the line current. It is a rectifying effect, and before the pressure again reverses, the arc vapor in the gaps has cooled to a non-conducting state.
Effect of Frequency.—The higher the frequency of the lightning oscillation, the more readily will the multi-gap respond to the pressure.
Briefly stated, the problem is to properly limit the line current so that the arc may be extinguished; to arrange a shunt circuit so that the series resistance will be automatically cut out if safety demand it on account of a heavy lightning stroke and, while retaining these properties, to make the arrester sensitive to a wide range of frequency.
It should be noted that series resistance limits the rate of discharge of the lightning as well as of the line current. The greater the value of the line current, the greater the number of gaps required to extinguish the arcs.
Graded Shunt Resistances.—Any arc is unstable and can be extinguished by placing a properly proportioned resistance in parallel with it. All the minor discharges then pass over the resistances and the unshunted spark gaps, the resistance assisting in opening the line current after the discharge.
Very heavy discharges pass over all the spark gaps, as a path without resistance, but those spark gaps which are shunted by the resistance, open after the discharge.
The line current, after the first discharge is accordingly deflected over the resistances, and limited thereby, the circuit being finally opened by the unshunted spark gaps. The arrangement of shunted resistances is shown in fig. 2,358.
