@misc{1154366, author = {Helmut Hellwig}, title = {Frequency standards and clocks ::a tutorial introduction}, year = {1972}, month = {1972-01-01 05:01:00}, publisher = {, National Institute of Standards and Technology, Gaithersburg, MD}, doi = {https://doi.org/10.6028/NBS.TN.616}, language = {en}, }