1. A MOLECULAR BEAM FREQUENCY STANDARD COMPRISING MEANS SUPPLYING A BEAM OF CARBON MONOXIDE MOLECULES CONTAINING A SUBSTANTIALLY GREATER NUMBER OF MOLECULES IN A FIRST ENERGY STATE THAN IN A SECOND ENERGY STATE, A GENERATOR HAVING AN OUTPUT FREQUENCY CORRESPONDING TO THE FREQUENCY OF THE TRANSITION BETWEEN SAID ENERGY STATES, MEANS FOR APPLYING THE OUTPUT OF SAID GENERATOR TO SAID BEAM TO CAUSE MOLECULES IN THE FIRST STATE TO UNDERGO A TRANSITION TO THE SECOND STATE, A DECTOR HAVING AN ALECTRICAL OUTPUT SIGNAL WHICH IS A FUNCTION OF A NUMBER OF MOLECULES UNDERGOING SAID TRANSITION, AND MEANS RE-