Quartz
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It is amazing what
changes have taken place in my lifetime. In 1929 radio was in its infancy but
it was growing rapidly and many amateur enthusiasts were starting to transmit
under Post Office licence. The frequencies of their transmitters were so poorly
controlled that they caused a lot of interference with other services. There
was no easy way for them to check the frequencies and the PO asked the NPL to
operate a transmitter in the amateur band to serve as a standard. This fitted
in well with its wider responsibility of establishing standards of measurement.
The frequency of
an oscillator is obtained by counting the number of cycles in a second; the
second was established by pendulum clocks at the Observatory which were set so
that they remained in step with the rotation of the earth and in effect divided
the day into equal seconds. The problem was to count the frequencies of many
thousands of cycles per second in terms of the 1-second ticks from the
pendulum. This was of course well before the days of the electronic counter. It
was much easier to compare one frequency with another so what was needed was a
standard of frequency which could be measured continuously in terms of the
pendulum and a means of multiplying its frequency giving a series of standards
throughout the range of use.
Tuning Forks
The man in charge
of this work was D.W. Dye a brilliant but rather irascible scientist who, it
transpired, had just had a row with his assistant and therefore needed another
pair of hands which I provided. He was investigating two forms of standard, a
tuning fork and a quartz oscillator. The tuning fork was delegated to me under
his supervision while he concentrated on the quartz oscillator.
The fork was cut
to have a frequency of 1000 cycles per second (now 1000 Hertz, a stupid
terminology) and was kept vibrating continuously by making use of its magnetic
properties. The prongs were placed near to coils attached to a magnet and the
signals induced in them were fed into a valve circuit. The oscillations in the
circuit were amplified and drove a small motor with a rotor having 100 teeth to
divide the frequency to 10 Hz and then to 1 Hz. This phonic motor designed by
Dye was the forerunner of the mains clock. The ticks from the motor were
compared continuously with those from a pendulum clock at the NPL which was
itself checked against observatory time signals. My contribution was to improve
the design of the fork and to install it in an airtight temperature controlled
chamber. As a timekeeper it was about as good as the pendulum itself.
Dye was a bachelor
who spent most of his spare time at work, much of it in the vaults of Bushy
House where our laboratory was situated. The advantage of the vaults was the
very steady temperature because there was then no temperature control in the rooms
– I keep having to insert these explanations of what might seem to be odd
behaviour. There was also very little ventilation there and this might have
been a contributory factor to his death from pneumonia three years after I
joined the NPL. With his death, interest in his work suffered a setback and a
small special grant made by the Radio Research Board was not renewed on the
grounds that the accuracy already achieved was adequate for the foreseeable
future. None of the other senior scientists in the department showed any
interest as they were immersed in their own lines of work, so that I was left
to do as I liked. My first task was to prepare for publication a description of
the tuning fork, an essential part of our work which I always find irksome. Presumably
it was completed satisfactorily since it was accepted by the Royal Society.
In addition to the
research, junior scientists were expected to carry out a certain amount of
testing of commercial instruments. The test work was always given priority.
This work worried some of us, but I rather enjoyed it myself. It acted as a
relaxation from the research especially if this was not going well, and made
one extremely careful and thorough, no bad training for a scientist.
Quartz
Dye had kept his
investigations on quartz much to himself but I had helped with an optical study
by interferometry of the complex vibrations of quartz plates. For this purpose
the faces of the quartz plates were ground and polished flat to a wavelength of
light, the mechanic in the optics workshop being very broadminded in
instructing us and letting us use his tools. This familiarity allowed me to
make the first oscillator on my own, a high frequency quartz plate to control
the standard transmissions, sent out once a week on an old transmitter in the
radio department. It was operated by a radio engineer with me controlling the
frequency. We were advised to work with one hand in our pocket so that we could
not get a shock across the body if the insulation broke down.
The frequencies
used in radio were becoming higher all the time and it was obvious that it
would be an advantage if the frequency of the standard was also increased and
that in spite of the success of the tuning fork, standards in the future would
be of quartz. Quartz is a remarkable mineral occurring naturally in large
crystals weighing several kilograms. At least they did, although the large
crystals have now been used up, and it is necessary to grow even small crystals
artificially. It has excellent mechanical properties and also the property of
piezo-electricity. If mechanical pressure is applied across the faces of a
quartz plate an electrical voltage appears on the other faces. These properties
enable a suitably cut piece of quartz to be maintained in oscillation in a
valve circuit. It is however a complicated crystal having different mechanical
and electrical characteristics in different directions.
Dye was
responsible for a fundamental investigation of these properties. He had
designed and cut an oscillator in the form of an annular ring 10cm in diameter,
mounted in a system of electrodes which constrained it to expand and contract
as a whole, the idea being to obtain the most uniform vibration possible. The
snag was that there were no nodes at which it could be held and it was
suspended on three fine metal wire stirrups designed to offer the minimum
damping. I set it up in an evacuated, temperature controlled oven on stable
platform in the basement of Bushy House as he had planned. Its frequency of
20kHz was divided to 1kHz by a circuit specially made for the purpose and this
in turn drove a phonic motor driving the hands of a clock dial to form a quartz
clock. It went for several years with a stability which was not easy to assess
as it was at least as good as the pendulum. I made two more with some
improvements to the mounting, one for experimental purposes and one for the
Australian Post Office; but I realised that the conditions of mounting and
temperature control were far too exacting for general use and I was already
experimenting with a much simpler form of oscillator.
Essen rings
The annular ring
possessed a number of advantages that were worth preserving. For example – and
this is not so far fetched as it might sound – molecules escaping from the surface
and reducing its size would 
not change the frequency which depends on
the mean diameter. The difficulty of support was overcome by using a form of
electrodes which made the ring vibrate in an overtone mode with six nodes round
the circumference, and a suitable choice of the width of the annulus gave it a
zero temperature coefficient. Finally the mean diameter was reduced to about
5cm, increasing the frequency to 100kHz. This new standard was not only much
easier to install than Dye’s ring but it proved to be a much better clock than
the pendulum at the NPL, or the time signals from the observatories. In all I
made six of them including one for the Royal Greenwich Observatory.
Unfortunately the latter was at their request adjusted to keep sidereal time
and not mean solar time, which limited its usefulness.
In parenthesis it
is interesting to recall the informality of the administration at that period.
The superintendent of our department was persuaded that the quartz ring clock
would be an improvement on the pendulums as an observatory time standard. He
arranged for me, a junior member of the laboratory, to meet the Chief Scientist
of the Navy, who then gave the Observatory the go ahead to order one from us.
Nowadays there would be four tiers of administration to pass through, each one
introducing delays and possible misunderstandings.
The Post Office
was interested in making quartz oscillators for its own requirements and had
built at its research station a modern workshop for the purpose. They designed
a quartz plate as a standard and supplied one to the Observatory. It was
described at a packed meeting at the Institution of Electrical Engineers at
which the Astronomer Royal spoke congratulating the PO on their achievement.
The mutual back slapping was rather overdone I thought, so, plucking up my
courage, I got up and suggested that the results were disappointing as the new
standard was much less stable than my NPL ring. The intervention was effective
because, although nothing was said at the time, the PO made some of the rings
using their better facilities to modify the mounting making it more portable.
Neither the PO nor the NPL could market it directly but arrangements were made
with a commercial firm to incorporate it with associated circuits and to sell
the complete quartz clock outfit. It was acquired by most of the world’s
leading observatories, including the RGO and the US Naval Observatory, where
their scientist in charge of timekeeping, Wm. Markowitz, was particularly
appreciative, having a model on display and stating that it had improved their
time signals by a factor of 10. As a civil servant I gained no financial
benefit – not even an extra promotion – but it was good for my reputation and
ensured me a welcome at observatories and laboratories throughout the world. My
direct interest in quartz standards stopped at this point. World War 2 had
started and I was transferred to the Radio Division which was more closely
concerned with research for the military departments.
