1. A CESIUM-BEAM FREQUENCY STANDARD COMPRISING: A MICROWAVE CAVITY RESONATOR HAVING A MICROWAVE ENERGY INPUT PART AND AN APERTURE THEREIN; MEANS PRODUCING A CESIUM BEAM PASSING THROUGH SAID APERTURE IN ENERGY COUPLING RELATIONSHIP TO THE MICROWAVE SIGNAL IN SAID CAVITY; AN IRIS DISPOSED AT SAID INPUT PART OF THE CAVITY TO ALTER THE RESONANT FREQUENCY THEREOF IN RESPONSE TO THE POSITION OF SAID IRIS; SAID IRIS INCLUDING A CONDUCTIVE MEMBER WHICH RESPONDS TO CHANGES IN TEMPERATURE TO ALTER POSITION FROM A NORMAL POSITION RELATIVE TO THE CAVITY RESONATOR FOR MAINTAINING THE RESONANT FREQUENCY OF SAID CAVITY CONSTANT WITH TEMPERATURE; AND MEANS FOR COUPLING MICROWAVE SIGNAL THROUGH SAID IRIS INTO SAID CAVITY.