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4. Operation
4.1 Safety instructions
The
diode
laser
(U22000)
requirements for class-II lasers. When used properly,
experiments can be carried out using the diode laser
with no risk.
Follow the instructions in the laser's instruction
manual.
Do not look directly into the laser beam or any reflected
beam.
Do not exceed the limits for laser dazzle, i.e. none of
the viewers should feel dazzled by the beam.
4.2 Note concerning where the experiment takes
place
The capacitive differential sensor largely suppresses
components of noise and vibration in the signal.
Nevertheless, it is essential to select a bench that
cannot be shaken. Serious disturbances (like the
slamming of a door) cannot be suppressed and give
rise to errors in the signal.
4.3 Installation of software and connection to a
computer
Before the apparatus is ready to use, the software
needs to be installed and the apparatus needs to be
connected to the PC or notebook on which the
program is running.
Insert the CD containing the software into the CD drive
of the computer.
Open Windows Explorer and navigate to the CD
drive. To start the install program, double click the
file "setup.exe" and follow the instructions
displayed by the install wizard.
Connect the torsion balance to the computer via the
USB cable.
Run the program.
If the installation has been carried out correctly, a
value displayed in milliradians will be shown in the
top left-hand corner of the window.
A detailed description of the software can be found in
Section 5.
4.4 Setting the measuring range
Take the glass plates out of their housing.
Screw the threaded aluminium rod into the middle
of the inner support beam, then assemble the
torsional balance in the housing, aligning it such
that the support beam is located centrally
between the two sensor boards.
Put the glass plates back in, connect to torsion
balance to the computer and run the software.
Click the program's "Setup" button.
Turn the support beam anti-clockwise by means
meets
the
safety
of its mounting until it touches the glass plate.
Click the "Adjust left border" button and confirm
by clicking "OK" to define the lower limit of the
measuring range.
Now turn the beam clockwise until it touches the
glass plate. This time, click the button "Adjust
right border" and confirm with "OK" to set the
upper limit of the range.
The design of the support beam includes lugs at the ends so
that it always oscillates within the linear range of the
capacitor plates.
The measurement software can pick up any
measurement value between +60 and -60 mrad,
although under normal circumstances the support
beam will only oscillate by a few mrad during the
experiment.
Figure 2 shows the dimensions of the inner support
beam. The maximum angle can easily be calculated
from these dimensions.
( ______ )
3 0 1 8 , 7
÷
−
α
=
150
2
2
18,7 mm
150 mm
Fig. 2 Dimensions of inner support beam
4.5 Assembly of torsional pendulum
Note: Assembling the torsional pendulum is difficult. It
requires a steady hand and good eyesight. The
tungsten wire is very thin and can easily break, so
that it needs to be treated with utmost care. In order
for the wire to be seen easily, it is recommended that
the torsional pendulum be assembled against a
background of white paper.
Place the torsion balance on a bench that cannot be
shaken and align it horizontally.
Screw in the lower suspension rod in the middle
of the inner support beam, taking care that the
surface of the mirror remains parallel to the
support beam (see Fig. 3).
Use a small screwdriver to loosen the screws attaching
both suspension rods.
Fix the upper rod to a piece of white paper using
adhesive tape to stop it rolling away.
Unroll about 8 to 10 cm of tungsten wire from its
bobbin but do not cut it off yet.
Fix the end of the wire to the paper next to the
upper suspension rod using a strip of adhesive
tape. Now wrap the wire around the fixing screw
and screw it in till it is tight, taking care to ensure
the wire does not get kinked. As far as possible,
the wire should run between the central axes of
the two suspension rods (see Fig. 3).
75 mrad
=
30 mm
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