Frequency standardization

The precision of frequency standardization increased rapidly after the introduction of mechanical oscillators maintained in continuous operation. A tuning fork gave an accuracy of 1 part in 106 in 1924, and about 10 years later quartz oscillations were being used with an accuracy approaching 1 part in 108. A critical analysis of the results obtained with quartz standards suggests that still higher frequency-stability could be obtained with improved mounting of the quartz and better control of the operating conditions. The N.P.L. ring oscillators appear to have given a very satisfactory performance in the important respect of long-term frequency stability, and a further development of these rings at the Post Office has yielded very promising results. The widespread distribution of the standard by means of radio transmissions has been extended and a detailed study made of the Doppler change of frequency caused by movements of the reflecting layers in the ionosphere. Early in the development of quartz oscillators it became clear that they were more reliable timekeepers than pendulum clocks and they are now used at the larger observatories as standards of time regulated to keep in phase with the earth´s rotation. The accuracy of time prediction and the uniformity of time signals has in consequence shown a marked improvement. This in turn facilitates the calibration of frequency standards. Extended observations with these precision clocks have made it clear that the period of the earth´s rotation is subject to variations, which will limit the precision of time and frequency measurements so long as they are based on the period of the earth´s rotation. Suggestions have been made that the velocity of light could be used as a standard, but the experimental accuracies of such methods are at present two or three orders lower than that of relating frequency to the mean solar second. Methods have been developed for measuring frequencies up to 50,000 Mc/s in terms of quartz st- andards; and also for controlling the frequencies of oscillators in this region by means of high-Q cavity resonators. These techniques have made it possible to investigate and measure the microwave absorption spectra of various gases. A resonance at 23,870 Mc/s in ammonia gas has been used to control the frequency of a quartz oscillator in an equipment which has been described as the first atomic clock, and other methods of using molecular and atomic resonances are being explored. Although the accuracy so far achieved is less than that of existing quartz clocks the foundation is being laid for an entirely new type of standard, which would be unaffected by the irregularities in the earth´s movement.