Vacuum-protective spark gap with trigger electrode

Claim 1

A PROTECTIVE SPARK GAP DEVICE FOR A.C. TRANSMISSION SYSTEM, INCLUDING A PAIR OF STATIONARY MAIN ELECTRODES INSULATED FROM ONE ANOTHER IN A CHAMBER UNDER HIGH VACUUM, ONE MAIN ELECTRODE BEING ADAPTED TO BE CONNECTED TO A HIGH-VOLTAGE A.C. POWER LINE AND THE OTHER MAIN ELECTRODE BEING ADAPTED TO BE CONNECTED TO EARTH, THE GAP BETWEEN THE MAIN ELECTRODES BEING SUCH THAT, WHEN THE MAIN ELECTRODES ARE SO CONNECTED, IT DOES NOT BREAK DOWN UNDER THE NORMAL LINE VOLTAGE, AND A PAIR OF STATIONARY TRIGGER ELECTRODES HAVING A CAPACITANCE BETWEEN THEM, EACH TRIGGER ELECTRODE BEING SPACED FROM THE CORRESPONDING MAIN ELECTRODE BY A TRIGGER GAP, THE TRIGGER ELECTRODES BEING ADAPTED TO BE CONNECTED EACH THROUGH A RESISTANCE TO THE HIGH-VOLTAGE POWER LINE AND TO EARTH RESPECTIVELY, AND THE TRIGGER GAPS BEING SUCH AS TO GIVE FIELD EMISSION IN VACUUM AT A VOLTAGE SUBSTANTIALLY LESS THAN THE MAXIMUM PERMISSIBLE LINE OVERVOLTAGE, THE ARRANGEMENT BEING SUCH THAT IF AN IMPULSE VOLTAGE APPEARS ACROSS THE GAP BETWEEN THE AIN ELECTRODES, IN ADDITION TO THE NORMAL LINE VOLTAGE, A CURRENT WILL FLOW THROUGH THE CAPACITANCE AND A SUBSTANTIAL PROPORTION OF THE IMPULSE VOLTAGE WILL APPEAR ACROSS THE RESISTANCES, AND THUS ACROSS THE TRIGGER GAPS, WHICH INITIATE BREAKDOWN OF THE GAP BETWEEN THE MAIN ELECTRODES.