NATURE May 7, 1955 Vol. 17
Table l. Variation of Frequency with Rotation of the Cavity Resonator
Date of measurement, December 3-15, 1954. Unit, 1 c./s. (1.09 parts in 1010)
Table 2. Variation of Frequency with Rotation of the Cavity Resonator, Individual Observations
Unit, 1 c./s. (1.09 parts in 1010)
observations were all made at about the same time of day. It will be noticed that there is a much larger periodic variation, having one maximum and one minimum per rotation. This was found to be due to a tilt of the turntable and was difficult to remove for reasons explained later. It was least in the last set of measurements, and the individual observations for this set (again corrected for linear drift) are given in Table 2 to illustrate the precision of measurement achieved.
The experiment was performed with existing apparatus and was completed within a few weeks of its assembly. Some of the practical difficulties that were met could probably have been overcome by the construction of special equipment, but others were of a more fundamental nature.
The cavity resonator was made of invar to secure the low temperature coefficient required in its original application5, and preliminary tests were therefore made to determine the effect of magnetostriction on the oscillator frequency. The changes of frequency observed on applying magnetic field-strengths between 1 and 10 gauss suggested that the earth’s field would produce a periodic change with rotation of about ±2 × 10–11, and this could account for the second harmonic term in the results given in Tables 1 and 2.
The turntable, which was adapted from the base of a fatigue testing machine by Messrs. H. L. Cox and N. B. Owen, carried the whole apparatus, including the resonator, the oscillator control circuits and the frequency standard, weighing altogether approximately 5 cwt. It was mounted on rollers and driven through a reduction gear by an electric motor; and a certain amount of gear vibration and non-uniform acceleration was detected. The effect of the vibration was reduced by mounting the oscillator on sponge rubber, but this accentuated the effect due to a tilt of the turntable to which reference has already been made.
The principal practical difficulty was, however, the adjustment of the oscillator to have the necessary small band-width and stability. Under the conditions accepted for observations, the band-width was such that a single comparison of frequencies in terms of the standard could be made with a precision of ± 1 part in 1010; but when the apparatus was rotating, this condition was not maintained for very long. If the stability of the oscillator could be increased, it might be worth while repeating the experiment with an improved turntable and with the magnetostriction effect eliminated by screening or by using a cavity resonator of different material.
It may be of interest to recall the most recent optical results. In the precise determination made by Joos6, the interference fringes were measured by ft microphotometer and it was concluded that there was a null result to ± 0.001 fringe or 0.3 per cent of the expected displacement. Miller7, however, who carried out the most extended measurements, was critical of this result and afterwards published a paper in which he persisted in his view that there is a definite effect of about 8 per cent of that anticipated. The present experiment suggests that Miller’s conclusions cannot be accepted; the effect, if any, is shown to be not more than one-tenth of that reported by him, which must probably be ascribed to some systematic error8.
The work described above has been carried out as part of the research programme of the National Physical Laboratory, and this paper is published by permission of the Director of the Laboratory.
1 Littman Furth, H., Nature, 173, 80 (1954).
2 Essen, L., Nature, 173, 734 (1954).
3 Littman Furth, H., Nature, 174, 505 (1954).
4 Crombie, D. D., Nature, 175, 350 (1955).
5 Essen, L„ Proc. Inst. Elect. Eng., 100, Pt. III, 19 (1953).
6 Joos, G., Ann. der Phys., 7, 385 (1930).
7 Miller, D. C., Rev. Mod. Phys., 5, 203 (1933).
8 Shankland, R. S., McCuskey, S. W., Leone, F.C., and Kuerti, G.,
Rev. Mod. Phys. (in the press).