1. A HIGH VOLTAGE ELECTRIC CIRCUIT BREAKER COMPRISING; AN OUTER CASING WHICH IS AT LEAST PARTIALLY FILLED WITH AN ARC-QUENCHING OIL, WALL MEANS OF INSULATING MATERIAL WITHIN SAID CASING DEFINING AN ELONGATED ARC BURNING AND QUENCHING CHAMBER WHICH IS OPEN AT ONE END, CURRENT LEAD-IN MEANS EXTENDING INTO SAID CHAMBER FROM OPPOSITE SIDES THEREOF AND HAVING NORMALLY INTERENGAGED CONTACT PIECES THEREON ADJACENT THE OPEN END OF SAID CHAMBER, SAID CONTACT PIECES BEING ADAPTED TO BE SEPARATED AND WHEN SEPARATED FORMING A GAP THEREBETWEEN IN SAID CHAMBER AND DRAWING AN ARC THEREBETWEEN ACROSS THE SAID OPEN END OF SAID CHAMBER, A PLATE-LIKE MEMBER OF INSULATING MATERIAL BETWEEN SAID WALL MEANS SUBSTANTIALLY IN THE PLANE OF SAID GAP NORMALLY SPACED FROM SAID CONTACT PIECES AND MOVABLE TOWARD SAID CONTACT PIECES AND THROUGH SAID GAP AND INTO SAID CHAMBER SO AS TO LENGTHEN AN ARC IN THE GAP BETWEEN SAID CONTACT PIECES, AND ACTUATING MEANS FOR MOVING SAID PLATE-LIKE MEMBER TOWARD SAID CONTACT PIECES AND THROUGH SAID GAP AND INTO SAID