1. IN COMBINATION WITH AN ELEMENT OF SUPERCONDUCTIVE MATERIAL IMMERSED IN A COOLING MEDIUM HAVING A TEMPERATURE LOWER THAN THE CRITICAL TEMPERATURE OF SAID MATERIAL; MEANS FOR FORMING AN INITIAL REGION OF NORMAL RESISTANCE IN SAID ELEMENT SEPARATED FROM THE SUPERCONDUCTIVE PORTION BY AN INTERFACE; MEANS FOR GENERATING SUFFICIENT HEAT IN SAID INITIAL REGION TO RAISE THE TEMPERATURE OF SAID SUPERCONDUCTIVE PORTION TO THE TRANSITION TEMPERATURE, WHEREBY SAID INTERFACE PROPAGATES DUE TO HEATING; MEANS FOR PRODUCING A FIELD TO GRADUATED INTENSITY ALONG THE SURFACE OF SAID ELEMENT TO STABLIZE SAID INTERFACE AT AN EQUILIBRIUM POSITION IN THE QUIESCENT CONDITION; AND MEANS RESPONSIVE TO AN INPUT SIGNAL FOR PERMITTING FURTHER PROPAGATION MOTION OF SAID INTERFACE PROPORTIONAL TO SAID SIGNAL.