Evening star., 1951-01-12
Atomic Clock Developed
At Bureau Of Standards
Varies Less Than q Second
In Three Years, Tests Show
By Thomos R. Henry

In lhree Tears, Tests Show
By Thomas R. Henry
An atomic clock which potentially will
keep time 600 times more accurately
than the ammonia molecule clock intro
duced two years ago, which has been
perfected to the point where it loses
or gains less than second in three
years, now is being developed at the
United States Bureau of Standards.
Ideally the ammonia clock should be
accurate to within less than a second
in 10000 years, but the technical prob
lens involved are such that this goal
can be approached only gradually and
probably never can be quite reached.
The new clock ideally should keep time
with a precision of about seconds
loss in million years. Here also tech
nical problems presumably always will
keep attainment far short of the ideal.
This new device is the caesium beam
clock. The rare element caeSium, a
silvery metal which now has limited
use in the radio tube industry, is heated
until it becomes vaporized. This vapor
is shot as a narrow beam through a
copper tube to a hot tungsten plate
When this plate is struck by tungsten
atoms vibrating at precisely the right
frequency, slight electrical change Is
produced. From this point on the de-
velopment becomes extremely compli-
Gated, but the net result is that time
can be measured theoretically to the
rate OF the atom vibration.
DR.. Lyons Directs Work.
The new "clock" is being developed
under the direction of DR.. Harold Lyons
of the Bureau staff, who also developed
the ammonia clock.
The device depends great deal on
the peculiar properties of caesium. The
molecule Of this element consists of a
single atom. Each atom is magnetized
particle which makes it possible to con-
centrate the vapor in a narrow beam
with all the particles moving through
the tube in the same direction and at
the same speed.
In the ammonia clock a radiation
beam. whose frequency is regulated by
a vibration quartz crystal, is sent
through a tube filled with ammonia
gas. The interference with this beam
if the vibration happens to be off, de-
ternines the extent to which the quartz
crystal is in error and Immediately ap-
plies an automatic correction.
The great advantage of the caesium
clock, 1t is explained. is that it enables
measurements to be made with a much
more precise frequency. The ammonia
molecule consists of several atoms. The
gas in the tube moves helter-skelter IN
all directions, introducing the so-called
Doppler effect. 1t cannot be pulled by
magnets into a sharp beam.
Although the ideal is far from at
tainment, DR.. Lyons explains, the whole
field of atomic time-keeping is in it's
infancy and can be expected to grow
better constantly with further research.
At least until the development Of
the Bureaus ammonia clock the primary
time standard was the period Of the
earth's rotation which is measured with
extreme accuracy at the Naval Observ-
atory by determination of the passing
Of certain stars over the zenith at
Washington. This still Is the official
standard
The precise time of passage of a star
is used automatically to correct the rate
OF vibration of quartz crystal kept in
an underground vault at the Bureau of
Standards. This vibration remains re-
markably constant through the "a
hours. N...E T-@F---r.
Drawbacks to System.
There are two drawbacks to this
system. First, the earth is slowing down
second or so a century due to the
braking effect of tides on shallow sea
bottoms. The greater part of this effect
comes from the tides of the Bering
Sea between Alaska and Siberia It also
recently has been determined that the
earth rotates at different rates in diff
ferent seasons. probably due to chang
ing turbulence in the planets atmos-
phere. A spring day is about thousandth
OF second longer than a fall day.
There also are spasmodic jumps and
drops OF as much as one part in
rs million in the speed Of rotation due
to causes for which there is no expla-
nation.
Less than generation ago these
inaccuracies would have been of no
consequence; but. with the DEVELOPMENT
OF various radio techniques, they threaten
eventually to be serious sources of error.
The vibration of the atom, on the
other hand, has no variation. It was
the same at the beginning Of time and
will remain so to the end.
Among the projects now planned by
the Bureau OF Standards physicists Is
the first absolutely accurate measure
OF the rotation of the earth itself, to
determine not only how much the rate
varies from year to year and season
to season but throughout an average
day. The planet may be whirling around
faster at noon than at midnight, for
all anybody knows.
The ammonia and the caesium clocks
are two OF four types of atomic clocks
which have been proposed, Bureau
physicists explain.
nnoe<sane -l A