@misc{1154191,
  author = {Helmut Hellwig},
  title = {Status report on primary frequency standards:},
  year = {1973},
  month = {1973-01-01 05:01:00},
  publisher = {, National Institute of Standards and Technology, Gaithersburg, MD},
  doi = {https://doi.org/10.6028/NBS.TN.646},
  language = {en},
}