1. A SENSITIVE CIRCUIT BREAKER COMPRISING IN COMBINATION: A NORMALLY OPEN MAGNETIC REED SWITCH; A SOLENOID SURROUNDING SAID SWITCH; A NON-MAGNETIC FRAMEWORK MOUNTING SAID SWITCH AND SOLENOID; A PERMANENT MAGNET MOVEABLY MOUNTED TO SAID FRAMEWORK IN A POSITION SPACED FROM SAID SWITCH AND SOLENOID TO DEFINE AN AIR GAP CAPABLE OF VARIATION BY MOVEMENT OF SAID MAGNET, SAID MAGNET BEING INF FLUX COUPLING RELATIONSHIP TO SAID SWITCH THROUGH SAID AIR GAP, AND SAID FLUX BEING SUFFICIENT TO CLOSE SAID SWITCH WHEN SAID SOLENOID IS DE-ENERGIZED; MEANS FOR MOVING SAID MAGNET WITH RESPECT TO SAID SWITCH TO A GIVEN POSITION TO CHANGE SAID AIR GAP AND THEREBY TO DECREASE SAID FLUX TO A VALUE JUST SUFFICIENT TO HOLD SAID SWITCH CLOSED WHEN SAID SOLENOID IS DE-ENERGIZED, SO THAT A GIVEN CURRENT IS REQUIRED IN SAID SOLENOID TO OPEN SAID SWITCH; AND STOP MEANS INCLUDED IN SAID FRAMEWORK DEFINING SAID GIVEN POSITION WHEREBY SAID MAGNET MAY BE CONSISTENTLY REPOSITIONED TO SAID GIVEN POSITION AFTER ANY MOVEMENT OF SAID MAGNET.