TY - GEN AU - A S Risley C2 - , National Institute of Standards and Technology, Gaithersburg, MD DA - 1969-01-01 05:01:00 DO - https://doi.org/10.6028/NBS.TN.399 LA - en PB - , National Institute of Standards and Technology, Gaithersburg, MD PY - 1969 TI - The physical basis of atomic frequency standards: ER -