Evening star., 1926-12-14
HAVE NEW DEVICE
To MEASURE TIME
Bureau Perfects Apparatus
of Minute Accuracy, Set
ting Record.

Apparatus for measurement of time
70 an accuracy of one or~two hundred
thousandths of second has been con-
structed at the Bureau Of Standards.
This is believed to be ten times as
fine Il measurement as has been at
tanned by any previous method. One
hundred thousandth of a second comes
very close indeed to the vanishing
point of time, SO far as human recog-
milton is concerned. But it may be-
come extremely important in some
physical measurements whose value
depends upon close approach to alb
solute accuracy. For instance light,
the yardStick of the universe, would
move nearly 2 miles in such an in
terval.
No clock or watch could even ap-
proach such accuracy. Some very
finely adjusted timepieces, it was eX-
planned at the bureau, might run over
the course of a day without losing or
gaining as much as second, but
when it came to measuring the Sec
onds themselves they would be found
to vary considerably. They might av
erage up rather accurately within the !
course of an hour or "a hours, but !
this would mean little to the scientific |
worker obliged to get inside of the !,
second itself in his researches.
Time Movements Used.
This new method of measuring time |
has five essential parts-a tuning |
fork, pendulum swinging in a |
vacuum, a photo-electric cell. an os-
cilograph or tiny mirror mounted on |
two wires in a strong magnetic field |
and a high-speed camera.
The process starts with the pendu- |!
1um. This is suspended and started |
swinging inside a case from which |
the air has been pumped so that a |
fair vacuum is created. 1t Is placed |
on bearings adjusted so nicely that |
friction is at a minimum. Since the
friction of the air has been removed
with the vacuum pump there is very |
little to stop this pendulum swinging |
Indefinitely once it has been started.
AS a matter of fact, it will swing for |
five hours, each complete swing TAX
ing half a second. i
Each time the pendulum in its |
swing passes definite point a fash
of light is let through q window in the |
case. This light flash falls upon a
photo-electric cell placed just behind
this window. In this cell the light
generates a tiny current of electricity
with every f1ash--a current of not
more than two or three niillionths of
generated every half second.
The current passes through three
stage vacuum tube amplifier where
1t is magnified about 10000 times, and
from there passes to the oscilograph,
where it causes the mirror to vibrate
at the same intervals as the swing of
the pendulum. Il
Attached to the apparatus is a tun-
ing fork vibrating at the rate of a
fba,rSand x,ihrafionC SQcond III
Vibrations Recorded.
Now the camera is so placed that
the film is exposed to both the mirror
vibrations and the tuning fork vibra-
tions. This film is 40 feet long, and
the camera is electrically operated.
Upon this film each vibration of the
tuning fork causes thin, black line
to appear at regular intervals. But
the film also is exposed to the mirror,
vibrating far more slowly. These
vibrations cause heavier lines to ap.
pear at intervals.
Thus the number of thin lines be-
tween two heavy lines represents the
number of times the tuning fork has
vibrated in half second. Each OF the
thin lines represents a thousandth of
q second and measurements of the II
space between them can easily be
made which will carry the time divi- |,
sion into hundred thousandths Of L
second.
The tuning fork and the pendulum
act as checks on each other. The ac-
curacy of each could be tested, de-
pending upon which was agreed upon L
as the standard. Thus ii 1t "was
agreed that the pendulum was CON-
pleting it's swing in exactly half
second and the tuning fork was
shown by the film to be vibrating /
only $50 times in this period, the fork
could be set down as inaccurate. On i
the other hand 1t might be assumed
that the fork was vibrating exactly
! 200 times in second and the pendu
lum would be at fault.
Use Special Forks.
Tuning forks can be made with |
varying vibratory rates and extremely |
accurate time measurements could |
be made with an instrument vibrating
only 100 times a second instead Of a
thousand. The behavior of a tuning |
fork still is rather obscure, according |
to p. p. Quale, of the sound labora-
tory, who has been in charge of con-
structing the apparatus and much re-
mains for scientists to find out. For !
instance. the vibrations will vary /
considerably with temperature |
changes-just why is uncertain.
Among the scientific researches |
which may be greatly aided by the |
extremely accurate time measure /
ments made possible by this method |
are the gravity measurements of the |
Coast and Geodetic Survey. But the |
great value at present will be in de-
termining the vibratory rate of vari- |
Gus tuning forks. L
Formerly this has been done with |
extremely accurate chronometers, but |
It has been possible by this method |
only to strike an average over a con-
siderable period-not to get inside |
Of the second itself