Image: Early NBS crystal oscillator frequency standards

Description: 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.
Title: Early NBS crystal oscillator frequency standards
Credit: Downloaded January 18, 2013 from Lombardi, Michael A.; Thomas P. Heavner; Steven R. Jefferts, . "NIST primary frequency standards and the realization of the SI second", NCLSI Measure: Journal of Measurement Science (NCSL International, Boulder CO, USA), Vol.2, No.4, p.77, fig.3 on A brief history of atomic clocks at NIST, Time and Frequency Division, NIST website Disclaimer on first page: "This paper is a contribution of the United States government and is not subject to copyright"
Author: Unknown
Usage Terms: Public domain
License: Public domain
Attribution Required?: No