Four precision 100 kHz quartz crystal oscillators maintained by the US Bureau of Standards (now the National Institute of Standards and Technology that served as the frequency standard for the United States in 1929. Built by Bell Telephone Laboratories, where the quartz crystal oscillator was invented in 1923, they achieved a frequency stability of 10^-7. The oscillators are enclosed in temperature controlled ovens kept at a precisely constant temperature to prevent thermal expansion and contraction of the quartz resonator, which would cause changes in frequency. The large crystal resonators are mounted under the glass domes visible on top of the units.