1. IN A CIRCUIT BREAKER HAVING A BASE, A STATIONARY CONTACT MOUNTED ON SAID BASE, A MOVABLE CONTACT MOVABLE IN A PREDETERMINED PLANE OF MOTION IN AND OUT OF ENGAGEMENT WITH SAID STATIONARY CONTACT TO INTERRUPT THE FLOW OF CURRENT. THERETHROUGH AND A COVER REMOVABLY SECURED TO SAID BASE, THE IMPROVEMENT COMPRISING AN ARC CHAMBER CLOSELY SURROUNDING SAID STATIONARY AND MOVABLE CONTACTS, SAID ARC CHAMBER INCLUDING: A PAIR OF INSULATING MEMBERS EACH PROVIDED WITH A GROOVE WHICH COOPERATES WITH AN INTERGRAL RIB ON SAID BASE TO REMOVABLY ALIGN SAID MEMBERS ON SAID BASE ON OPPOSITE SIDES OF SAID CONTACTS, EACH OF SAID INSULATING MEMBERS HAVING INTERIOR SURFACES EXTENDING PARALLEL TO THE PLANE MOTION OF SAID MOVABLE CONTACT, SAID INTERIOR SURFACES BEING SPACED FROM ONE ANOTHER BY A DISTANCE WHICH IS JUST SLIGHTLY LARGER THAN THE WIDTH OF SAID MOVABLE CONTACT; A FIRST INSULATING SHEET DISPOSED BETWEEN SAID INSULATING MEAMBERS AND SAID BASE, SAID FIRST INSULATING SHEET HAVING AN APERTURE THROUGH WHICH A MOVABLE-CONTACTENGAGING PORTION OF SAID STATIONARY CONTACT MAY PASS INTO SAID ARC CHAMBER; SAID FIRST INSULATING SHEET BEIN SECURELY HELD IN PLACE BY SAID INSULATING MEMBERS WHEN SAID COVER IS PLACED ON SAID BASE; AND A SECOND INSULATING SHEET SECURED TO SAID MOVABLE CONTACT AND MOVABLE THEREWITH, SAID SECOND INSULATING SHEET HAVING AN APERTURE THROUGH WHICH A STATIONARYCONTACT-ENGAGING PORTION OF SAID MOVABLE CONTACT MAY PASS INTO SAID ARC CHAMBER; SAID INTERIOR SURFACE OF SAID INSULATING MEMBERS AND SAID FIRST AND SECOND INSULATING SHEETS COOPERATING TO DEFINE A SMALL ARC CHAMBER WHICH CLOSELY SURROUNDS SAID STATIONARY AND MOVABLE CONTACTS TO AID ARC EXTINGUISHMENT WHEN SAID STATIONARY-CONTACT-ENGAGING PORTION OF SAID MOVABLE CONTACT IS DISENGAGED FROM SAID MOVABLE-CONTACT-ENGAGING PORTION OF SAID STATIONARY CONTACT.