The redefinition of the second and the velocity of light

A very popular session at the International Precision Electromagnetic Measurements Conference was the Thursday night meeting organized to discuss the proposed atomic definition of the second and methods for measuring the velocity of light. This discussion was moderated and stimulated by Professor Ramsey, who entertained the participants on numerous occasions throughout the two‐hour meeting with remarks such as his invitation to proponents of the pendulum clock to speak up, and with his method of smoothing over a somewhat semantic debate concerning either the proper qualifications to be furnished with a definition of the second or the question as to whether such qualifications are or are not part of the definition. The dialogue and response to Ramsey’s balm went more or less as follows:

(Ramsey, addressing the debators) “I believe I agree with both of you. Let us suppose that in 1966, or before, that some transition in some material, let us say x or—”

(Unknown participant) “Hydrogen.”

(Ramsey) “Cesium.”

(Participants) Laughter.

(Ramsey) “Well, yes, cesium might well be recommended by the subcommittee. Then, following the recommended definition, there might appear a qualifying remark such as: ‘It is interesting to note that the frequencies of hydrogen and thallium transitions vary up and down together with respect to cesium by parts in ${10}^{12}$’.”