1. A SOLENOID RELAY COMPRISING A CORE FORMED WITH AN ELONGATED CHAMBER OF SUBSTANTIALLY UNIFORM TRANSVERSE CROSS-SECTION, A PISTON RECIPROCABLE IN AXIAL DIRECTION WITHIN SAID CHAMBER AND FORMING AN ARMATURE, A COIL SURROUNDING SAID CORE FOR ELECTROMAGNETICALLY DISPLACING SAID PISTON AXIALLY UPON ENERGIZATION OF SAID COIL, BIASING MEANS BEARING ON SAID PISTON AND EFFECTIVE IN A DIRECTION OPPOSITE TO THAT OF THE ELECTROMAGNETIC DISPLACEMENT OF SAID PISTON TO DISPLACE SAID PISTON IN SAID OPPOSITE DIRECTION UPON DE-ENERGIZATION OF SAID COIL, UNIDIRECTIONALLY EFFECTIVE VALVE MEANS IN SAID CORE OPERABLE UPON DISPLACEMENT OF SAID PISTON IN ONE OF SAID DIRECTIONS FOR SUBSTANTIALLY FREELY PERMITTING THE PASSAGE OF A FLUID OUT OF SAID CHAMBER, ADJUSTABLE THROTTLE MEANS ON SAID CORE EFFECTIVE UPON DISPLACEMENT OF SAID PISTON IN THE OTHER OF SAID DIRECTIONS FOR THROTTLING THE FLOW OF SAID FLUID INTO SAID CHAMBER, CONTACT MEANS SECURED TO SAID CORE, A CONTACTACTUATING MEMBER ENGAGEABLE WITH SAID CONTACT MEANS AND SLIDABLY MOUNTED ON SAID PISTON FOR AXIAL DISPLACEMENT RELATIVE THERETO BETWEEN TWO LIMITING POSITIONS, AND RESPECTIVE ABUTMENTS ON SAID PISTON ENTRAINING SAID CONTACT-ACTUATING MEMBER IN EACH OF SAID LIMITING POSITIONS FOR OPERATING SAID CONTACT MEANS, SAID CONTACT MEANS AND SAID MEMBER BEING PROVIDED WITH COMPLEMENTARY MEANS FOR RELEASABLY RETAINING SAID MEMBER INTERMEDIATE SAID LIMITING POSITIONS.