PASCO AP-8215A Gravitational Torsion Balance - Manual (Page 9)

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M o d e l N o . A P - 8 2 1 5 A E q u i p m e n t S e t u p
5
Rotational Alignment of the Pendulum Bob Arms (Zeroing)
The pendulum bob arms must be centered rotationally in the case —
that is, equidistant from each side of the case (Figure 7). To adjust
them:
1. Mount a metric scale on the wall or other projection surface that
is at least 5 meters away from the mirror of the pendulum.
2. Replace the plastic cover with the aluminum cover.
3. Set up the laser so it will reflect from the mirror to the projec-
tion surface where you will take your measurements (approxi-
mately 5 meters from the mirror). You will need to point the
laser so that it is tilted upward toward the mirror and so the
reflected beam projects onto the projection surface (Figure 8).
There will also be a fainter beam projected off the surface of the
glass window.
4. Rotationally align the case by rotating it until the laser beam
projected from the glass window is centered on the metric scale
(Figure 9).
5. Rotationally align the pendulum arm:
a.Raise the locking mechanisms by turning the locking screws
until both of the locking mechanisms barely touch the pendu-
lum arm. Maintain this position for a few moments until the
oscillating energy of the pendulum is dampened.
b.Carefully lower the locking mechanisms slightly so the pen-
dulum can swing freely. If necessary, repeat the dampening
exercise to calm any wild oscillations of the pendulum bob.
c.Observe the laser beam reflected from the mirror. In the optimally aligned system, the equilibrium point of
the oscillations of the beam reflected from the mirror will be vertically aligned below the beam reflected
from the glass surface of the case (Figure 9).
Case plates
Small mass
The pendulum bob arm must
be centered rotationally
between the plates.
Figure 7: Aligning the pendulum bob
rotationally
Top, Cutaway View
L L
Top View
Figure 8: Setting up the optical lever
Side View
Screen with
scale
Laser
Reflected beam
(from mirror)
0
Figure 9: Ideal rotational alignment
(zeroing) of the pendulum
Location of the
projected laser
beam from the
glass window
Location of the
projected laser
beam from the
mirror