Evening star. [volume] (Washington, D.C.) 1854-1972, 1931-03-27
All of Clocks in City Could
Be Operated From Bu-
reau of Standards.
PLAN TIME SIGNALS
FOR TESTING PURPOSES
Instrument Generates Enough
Electricity to Insure Con-
slant Operation.
BY THOMAS R. HENRY.

A time-keeping mechanism which it
is expected will operate with an ac-
curacy Of one ten-thousandth of a sec
ond is being set up at the Bureau Of
Standards.
The essential mechanism is a free
pendulum swinging in. an approximate
vacuum. To the top Of the swinging
pendulum is attached a mirror. Upon
the mirror falls beam Of light. Then
photo-electric cell is so placed that
with each swing Of the pendulum the
mirror reflects light upon 1t through a
tiny slit. This generates two or three
mtllionths Of an amprere of electricity
which passes through a one-stage am
plifier to high resistance relay.
From this point on the extremely ac-
curate timekeeping device can be at
tached to as many clocks as desired. It
Is intended to send accurate time sig
mails through the bureau laboratories
for testing purposes. For that matter,
all the clocks in the city could be OPP
Grated from this one pendulum. It Is
only matter of current adjudication.
There would be some slight loss in ac-
curacy with each extension, but the de-
vice would remain an extremely precise
b1iAS"AccpAuB ulSCIldulSlE.
Accuracy Due TO Simplicity.
The swing Of the pendulum is only
over distance Of ! centimeter through
l degree Of arc. It could be made much
shorter If any decrease in accuracy
could be gained by doing this. The ex-
IRENE accuracy of the pendulum is due
In part to the simplicity of the device
There are no complicated gears to pro-
duce friction.
The pendulum is kept swinging by
magnetized coil which IS controlled by
the pendulum itself First the pendu-
lum is set swinging. Each minute it
establishes a contact which sends a
slight current through the coil. The
coil, thus magnetized, exerts a slight
pull on the pendulum. which keeps it
swinging. So the motive power of the
mechanism is renewed every minute.
Inclosed In an evacuated brass cylin-
der, there Is practically no loss from
friction with the air.
The pendulum is kept in the base
ment of one of the bureau laboratories,
where it Is free from any vibrations of
the building and where a constant
temperature can be maintained. It is
set on concrete. pier resting on the
ground, so that the only vibrations to
which it Is subjected are those Of the
earth itself. The pendulum itself is
an invar rod whose coefdcient of ex-
pansion with temperature change, ac-
cording to r. z. Gould, who is setting
up the device, has been shown to be
100000. The calculation has only been
carried to seven decimal places The
brass bob Is attached to the pendulum
in such way that whatever expansion
takes place is up and down from the
center. so essentially the length Of the
pendulum never changes.
The mirror on the pendulum and the
slit through which the light falls on
the photoelectric cell are arranged so
precisely in relation to each other that
the light passes through the slit at ex-
actly the central point Of each swing
The central point Of the swing Of the
pendulum, the passage OF the light
through the slit, the starting of the
current and its amplihcation are eS-
sentially simultaneous, well within a
ten thouSandth of a second possibility
of error. Actually 1t Is probable that
the time device will function with con-
siderably greater precision, but that is
all Mr.. Gould claims for 1t.
Trial pendulums have been operated
successfully from a similar device set
up by Mr.. Gould, but with which the
same extreme precautions were not
taken but which indicated very high &C
curacy. The success Of the preliminary
work seems to warrant the more elebo-
rate construction now being installed.