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Claim 1

IN AN ELECTRICAL SYSTEM, A PAIR OF TERMINALS TO BE ENERGIZED IN ACCORDANCE WITH INPUT VOLTAGES EACH HAVING UPPER AND LOWER LEVELS INDICATIVE OF TWO DIGIT VALUES, SAID TERMINALS HAVING APPLIED THERETO AT ANY GIVEN TIME ONE OF FOUR COMBINATIONS OF INPUT VOLTAGE LEVELS, A FIRST ELECTRORESPONSIVE VALVE HAVING A FIRST CONTROL ELECTRODE AND A PAIR OF FIRST MAIN ELECTRODES, SAID FIRST CONTROL ELECTRODE BEING CONNECTED FOR ENERGIZATION IN ACCORDANCE WITH VOLTAGE APPLIED TO ONE OF SAID TERMINALS, A SECOND ELECTRORESPONSIVE VALVE HAVING A SECOND CONTROL ELECTRODE AND A PAIR OF SECOND MAIN ELECTRODES, A CIRCUIT MEANS CONNECTING SAID SECOND CONTROL ELECTRODE TO ONE OF SAID FIRST MAIN ELECTRODES AND TO THE OTHER OF SAID TERMINALS, SAID CIRCUIT MEANS BEING ARRANGED TO APPLY TO SAID SECOND CONTROL ELECTRODE A FIRST CONTROL VOLTAGE HAVING A FIRST LEVEL FOR ONE COMBINATION OF SAID INPUT VOLTAGE LEVELS AND HAVING A SECOND LEVEL FOR THE OTHER THREE COMBINATIONS OF SAID INPUT VOLTAGE LEVELS, A FIRST OUTPUT TERMINAL CONNECTED TO ONE OF SAID SECOND MAIN ELECTRODES HAVING APPLIED THERETO A FIRST OUTPUT VOLTAGE HAVING ONE LEVEL FOR SAID ONE COMBINATION OF THE INPUT VOLTAGE LEVELS AND HAVING ANOTHER LEVEL FOR SAID OTHER THREE COMBINATIONS OF THE INPUT VOLTAGE LEVELS, A THIRD ELECTRORESPONSIVE VALVE HAVING A THIRD CONTROL ELECTRODE AND A PAIR OF THIRD MAIN ELECTRODES, ADDITIONAL CIRCUIT MEANS CONNECTING SAID THIRD CONTROL ELECTRODE TO ONE OF SAID FIRST MAIN ELECTRODES TO ONE OF SAID SECOND MAIN ELECTRODES, AND TO SAID OTHER TERMINAL, SAID ADDITIONAL CIRCUIT MEANS BEING ARRANGED TO APPLY TO SAID THIRD CONTROL ELECTRODE A SECOND CONTROL VOLTAGE HAVING A FIRST LEVEL FOR ONE PAIR OF COMBINATIONS OF SAID INPUT VOLTAGE LEVELS AND HAVING A SECOND LEVEL FOR THE OTHER PAIR OF COMBINATIONS OF THE INPUT VOLTAGE LEVELS, A SECOND OUTPUT TERMINAL CONNECTED TO ONE OF SAID THIRD MAIN ELECTRODES AND HAVING APPLIED THERETO A SECOND OUTPUT VOLTAGE HAVING A FIRST LEVEL FOR ONE PAIR OF COMBINATIONS OF THE INPUT VOLTAGE LEVELS AND HAVING A SECOND LEVEL FOR THE OTHER AIR OF COMBINATIONS OF SAID INPUT VOLTAGE LEVELS, AND MEANS FOR APPLYING ENERGIZING POTENTIALS TO EACH OF SAID CIRCUIT MEANS AND SAID FIRST, SECOND AND THIRD VALVES.