University of Virginia Library

Gravitational Measuring Device
Developed By University Team

A device which will clarify a
two-and-a-half-century old scientific
puzzle has been developed
at the University.

Conceived by National
Medal of Science winner. Jesse
W. Beams, the device is being
used to pin down the exact
value of the gravitational constant,
or "big G."

In the 18th century, Sir
Isaac Newton first described
the gravitational forces with
which every particle in the universe
attracts every other particle dependent
on the mass of each and the
distance tween them. His mathematical
formula for this force
contains a quantity which he called
"G" and which he supposed never
changed. Because the gravitational
force is so weak. Scientists have
been able to measure G to only one
part in 500.

Greater Accuracy

Now the University ,
headed by M Beams, hope to
refine the measurement to one part
in 100,000. "nd once we get it to
that degree, we'll y for even
greater accuracy," says Mr. Beams.
professor of physics.
"Since enters into all cosmological
theories and because of the
recent emphasis on space . It
seemed we ought to increase your
knowledge of G."

A more precise knowledge of G
would, for example, enable scientists
to better calculate the density
of the earth, as well as the other
planets, says Mr. Beams

The new device surmounts some
of the obstacles which kept other
methods of measurement from
achieving greater accuracy the
elimination of the gravitational
effect of objects other than those
being measured and the measurement
of a weak force over a period
of time.

In order to eliminate the gravitational
effects of surrounding objects,
the University team devised a
system in which a turntable's
acceleration would be the quantity
measured.

Tungsten Spheres

Two high density tungsten
spheres of equal weight are fixed on
opposite sides of a turntable. A tiny
metal cylinder suspended by a
quartz fiber in an airtight, helium
filled chamber is placed in the
center of the turntable with the
cylinder's as at an acute angle to
the two tungsten spheres.

Each side of the cylinder is
attracted to the sphere closest to it
and it begins to rotate in an
attempt to close the angle between
it and the sphere. The turntable,
however, prevents the cylinder
from catching up to the sphere.

An optional tracking device
attached to the turntable activates a
motor which causes the table to
rotate as the cylinder begins to
move, keeping the angle between
the cylinder and the sphere constant.
As the cylinder keeps trying
to catch up to the sphere, the table,
which floats on a thing layer of air,
accelerates and within two hours is
making five revolutions per minute.

Angular Displacements

"The problem now is to measure
long angular displacements over
long periods of time, both of which
can be done with a high degree of
accuracy," says, Ralph A. Lowry,
chairman and professor of aerospace
engineering and one of the
University researchers.

The data on rotation speed is
fed into a computer which performs
the calculations required to
convert the speed into the units by
which gravitation is measured.

Since so many variables can
affect the measurements, precision
instruments in a controlled setting
must be used. The temperature of
the cubicle in the School of
Engineering and Applied Science
building where the device is located

never varies beyond of one
degree. White gloves are donned in
handling the tungsten spheres to
prevent soiling them.

"We've found that the best time
to take measurements is between
midnight and 4 a.m.," says Mr.
Lowry. "The vibrations from passing
cars and people are not as
much of a problem then."

Improvements on the device are
being made constantly in order to
increase its accuracy. For instance,
the quartz fiber suspension will be
replaced by magnetic suspension to
allow the cylinder to "float"
unhampered by friction and the
slight restoring torque of the quartz
fiber.

Other members of the research
group which designed and developed
the device under a grant from
the National Aeronautics and Space
Administration are Hermon M.
Parker, professor of aerospace engineering:
A.R. Kuhlthau, associate
provost for research and professor
of aerospace engineering; Maj. R.D.
Rose, assistant professor of physics
at the Air Force Academy who
received his doctorate from the
University in August, 1969, and
W.D. Towler, head of the instrumentation
group of the University's
Research Laboratories for the Engineering
Sciences.