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Now place the lower suspension rod on the paper
about 4 to 5 cm below the upper rod. Wrap the wire
around the fixing screw so that it is slightly under
tension and screw it in tight.
Now you can cut the wire off the roll.
Carefully suspend the torsional balance from a stand
by its upper suspension rod and check that it is
aligned horizontally.
To align the support beam for the torsional
pendulum horizontally, take it back off the stand
and carefully lay it on the bench. Undo the screw
for the lower suspension rod, hold the rod still
with your hand and move the beam as needed.
Tighten up the screw again and check the
alignment once more.
Fig. 3 Schematic diagram of torsional pendulum (the detail on the left shows a view from above of the support beam
with the lower suspension rod)
4.6 Setting the zero point
To set the zero point, it is recommended that the
following laser be used as a laser pointer:
Diode laser, red 650 nm
Place the torsion balance on a bench that cannot
be shaken about 2 to 3 m from a flat wall and
connect it to a computer (see Fig. 5).
Set up the laser so that the beam is at the same level
as the mirror on the lower suspension rod.
Observe the laser pointer on the wall.
Wait till the vibration of the torsional pendulum has
died down.
Turn the support beam in a series of steps of
small angles by means of the upper suspension
rod until it comes into contact with the glass
plates. A 45° graduated scale is attached to the
Once it is correctly aligned, carefully place the
torsion pendulum in its housing, setting the
support beam on top of the lower, green sensor
board, with the upper suspension rod passing
through the hole drilled in the top of the housing.
Adjust the height using the adjustment screw so that
the support beam can swing freely between the
sensor boards. The screw must be screwed against
the flat side of the suspension rod.
If the pendulum cannot swing freely, repeat the
above procedure for positioning the suspension
system on the support beam. Further correction can
be made using the adjustable feet.
Place the small lead spheres on the inner support
beam and insert the glass plates.
4 - 5 cm
housing around the suspension rod for your
assistance (see Fig. 4).
Observe the movement of the light pointer and
mark the maximum extent of the oscillation to
U22000
the left and right on a piece of paper fastened to
the wall.
Determine the centre position between the marks
for the two extremes and mark that on the wall,
too.
Turn the suspension rod until the support beam is
in this central position and wait for the pendulum
to stop vibrating in that position. Alternatively, the
support beam can be fixed in this central position
using the centring rod.
Set this position as the zero position in the
software.
Keep the set-up of the apparatus like this for the
course of the experiments.
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