[Boatanchors] The Crystal Clock

[William L Howard]

>From Mindy at Fort Monmouth:
Historic Firsts: The Crystal Clock


Frequency is of the essence of time, and is generally used as its
measure. One cycle of the axial rotation of the earth establishes our
basic temporal unit, the day. Smaller units of sidereal time are the
hour, the minute, and the second, derived by dividing the day into 24,
1, 440, or 86, 400 equal parts. A device making only one cycle per day,
however, is not convenient for timing ordinary affairs, and for nearly
three centuries the most accurate secondary marker of time has been the
pendulum. Through an escapement mechanism, the pendulum controls the
motion of hands on a dial to indicate time. Besides measuring time, such
a clock also measures frequency- the frequency of the oscillation of the
pendulum.

While frequency is thus important to the measurement of time, it is
important in its own right to electrical communication. The quality of
speech, the transmission characteristics of electrical circuits, and the
behavior of telephone transmitters and receivers all depend on the
frequencies involved. The measurement of frequency is thus basic to
telephone research, but just as the terrestrial cycle is too slow for
ordinary measurements of time, so the pendulum is too slow for ordinary
measurements of frequency. For many years Bell Laboratories employed a
100-cycle tuning fork as a standard. The oscillations of the fork also
were counted by a clock mechanism, but in this case the hands of the
clock were driven by a synchronous motor using an alternating current
whose frequency was governed by the fork.

Because of the ever present need for greater precision, and of the
increasing use of higher frequencies in radio and carrier systems during
the 1920�s, it was decided to use a quartz crystal as the oscillating
standard. After an extensive study of the factors involved, W.A.
Marrison devised and built a crystal-controlled clock. The clock was put
into operation in 1927, and from the very start proved a very accurate
timepiece- with a daily deviation of less than one part in one million.
Quickly improved by 1929 to one part in 10 million, the crystal clock
can be provided in greatly simplified form with a daily deviation of
well under one part in a hundred million.

For astronomical use, crystal clocks have a number of advantages in
addition to their great stability. Chief among these is their complete
freedom from the effect of gravity, and the facility with which a single
crystal can control mechanisms indicating both sidereal and mean solar
time, from either of which accurate time determinations can be made at
any instant. Because of these advantages, and because the accuracy of a
good crystal clock is now greater than that of the most precise pendulum
clocks, it has been widely adopted by astronomical observatories and
Government laboratories, where it is playing a role of steadily
increasing importance.