Evening star. [volume] (Washington, D.C.) 1854-1972, 1950-05-15
Light Is Used 10 Weigh
30-BilIionth of Ounce
Virginia U. Physicist
Describes Apporotus
By Thomas Z. Henry

cf lhomoS x. henV
CHARLOTTESVILLE, Va-Accurata
weighing Of q thirtieth OF q billionth Of
an ounee-the finest weight discrimina.
ton ever achieved-has been accom-.
plished in the physics laboratory OF the
University OF Virginia here.
It was reported last week before ths
Virginia Academy OF Sciences meeting
at Roanoke by DR.. Jesse w. Beams OF the
university faculty. DR.. Beams has at
tanned distinction as the inventor OF the
ultra-centrituSe whirling in near
vacuum at the greatest speeds known ex-
cept for the speed OF light.
The fine balance now reported by DR..
Beams uses for its arm a weightleSS beam
of light. 1t is suspended in near
vacuum by magnetism so there is no
friction. Theoretically there is no limit
to the capacity of the apparatus for fine
weight discrimination. even down to the
billion-fold greater SenSitivity that
would make possible the weighing OF a
single atom. Actually, however. Or.
Beams has reached about the limit pos
sible with the apparatus under present
conditions. At the best. ne anticipates,
he will not be able TO weigh A much
timer mass than 300th Of q billionth Of
an ounce.
No Supports Used.
A bit of steel weighing approximately
one milligram is suspended without sup
ports in vacuum by the magnetic at
traction of an electric current passing
through coil. A beam Of light is
focused from the side OF the apparatus
so that 1t just touches the top Of the
steel which, 1t has already been de-
termined weighs approximately one mil
ligram. Its exact weight, down almost to
the last atom, determines the distance
below the magnetic coil at which II re-
mains suspended.
The beam of light is reflected to a
mirror above the coil. II any OF 1t is cut
of by the steel weight less light Is re-
flected to the mirror. The light from
the mirror in turn is focused on a photo
electric cell to determine the amount OF
current passing through the coil The
variations in this current, which are re-
corded with extreme accuracy. determine
the minute variations in the weight of
the mass.
Once this is determined within a 30th
OF billionth of an ounce, other objects
can be suspended from 1t and their
weight determined in the same way.
Despite the theoretical infinite pos
sibility OF weight discrimination, DR..
Beams says. thousand fold, or even
hundred-fold finer measurements will be
extremely difficult because of the funda-
mental property of matter itself. All
matter Is made up of molecules. At
ordinary temperatures these are in rapid
motion.
This inability OF anything to remain
still for the tiniest fraction OF a second
disturbs the balance. 1t is like trying TO
weigh a. rabbit that keeps jumping up
and down on the scale. Moreover, o per-
feet vacuum Is unattainable. The finest
which ever can be reached on earth still
is millions Of times more dense than
outer space. and even this is far from
perfect emptiness. This means that the
air molecules surrounding the mass
being weighed are constantly humping
against its sides and kicking it back and
forth.
Can't Be Detected.
There would be no conceivable means
OF detecting such motions but 1t can be
proved that they exist and they are Suf-
f1cent to cause weighing errors as great
as Tooth of billionth Of an ounce
This can be partially overcome in
three ways, DR.. Beams believes. First. 1t
will be necessary to create a far more
perfect vacuum in the tube in which the
weight is suspended Second, II will be
necessary to reduce the temperature as
far as possible.
At absolute zero, -4596 Fahrenheit,
all molecular motion stops and matter
becomes completely dead. Then there
would be no internal vibration Of the
steel weight and no air molecules kick
ing it from the sides. But. the Virginia
physicist says. 1t will never be possible
to create an absolute vacuum or to reach
absolute zero. So there never can be
the complete lack of motion necessary
for perfect weighing.
Nearly as fine measurements have been
achieved with a quartz fiber but this
method has reached its limits while the
light beam balance still has great pos
sibilities of finer weight discrimination.
1t also is somewhat simpler TO use. and
will weigh heavier masses. In an atomic
age it has far-reaching potentialities for
usefulness, DR.. Beams explained. for
many vital reactions depend on mass
differences Of much less than q billionth
of an ounce.
The instrument described, 1t was ex-
planned is probably only a crude ap-
Proclamation of what can be attained
and the balance promises to be one OF
the most important additions ever made
to the tool chest of the atomic physicist.