In 1929 these four 100 kHz quartz oscillators, with auxiliary circuitry and temperature-control equipment, were installed to become the National Primary Standard of Radio Frequency. At this early date, frequency was derived from the difference of accurate measurements of mean solar time (from astronomical observations). These measurements were made regularly, because the quartz oscillators drifted in frequency by 1 part in 10⁷ per month.

Picture taken from Achievement in Radio, W.F. Snyder and C.L. Bragaw (US Gov. Printing Office, 1986).