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7
Attach to
Ear
th
Ground.
GRA
VIT
TORSION
B
Setting up for the Experiment
1.
Take an accurate measurement of the distance from the
mirror to the zero point on the scale on the projection
surface (
L
) (Figure 8). (The distance from the mirror surface
to the outside of the glass window is 11.4 mm.)
Note:
Avoid jarring the apparatus during this setup procedure.
2.
Attach copper wire to the grounding screw (Figure 11), and
ground it to the earth.
3.
Place the large lead masses on the support arm, and rotate
the arm to Position I (Figure 12), taking care to avoid
bumping the case with the masses.
4.
Allow the pendulum to come to resting equilibrium.
5.
You are now ready to make a measurement using one of
three methods: the final deflection method, the equilibrium
method, or the acceleration method.
Note:
The pendulum may require several hours to reach resting
equilibrium. To shorten the time required, dampen the oscillation
of the pendulum by smoothly raising the locking mechanisms up
(by turning the locking screws) until they just touch the crossbar,
holding for several seconds until the oscillations are dampened,
and then carefully lowering the locking mechanisms slightly.
Figure 11
Attaching the grounding strap to the
grounding screw
copper wire to
earth ground
grounding
screw
Figure 12
Large Masses:
Position I
Large Masses:
Position II
Mirror
Light beam
M
ea
su
r
i
ng
t
h
e
G
r
a
v
i
t
a
t
i
on
a
l
C
ons
t
a
n
t
Overview of the Experiment
The gravitational attraction between a 15 gram mass and a 1.5 kg
mass when their centers are separated by a distance of
approximately 46.5 mm (a situation similar to that of the
Gravitational Torsion Balance ) is about 7 x 10
-10
newtons. If this
doesn’t seem like a small quantity to measure, consider that the
weight of the small mass is more than two hundred million times
this amount.
case
glass
window
small
mass
Moving the large masses into Position I
Summary of Contents for AP-8215
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