3B SCIENTIFIC PHYSICS U8481350 Instructions D'utilisation page 9

tions, a saw-tooth generator is used to supply this
voltage. In this case, the beam goes from left to right
and then quickly flicks back again. This is repeated at
a fixed frequency that can be adjusted. By this means
it is possible to display vertical deflections that are
also periodic, such as an alternating magnetic field.
4.2.2 Magnetic deflection
Attach the coils to the magnetic ring surrounding the
neck of the tube. Between each neighbouring socket,
there are 300 turns so that if a connection is made to
the two outer sockets, the current flows through all
600 turns. The electron beam is deflected to the right,
perpendicular to the magnetic field and the direction
of the beam. If the coils are mounted facing inward,
even small currents of a few milliamps will be notice-
able.
4.2.3 Beam adjustment
A magnet is attached to the tube support in the mid-
dle that can be adjusted by a screw so that the beam
can be aligned to strike a specific point on the screen
when the deflecting apparatus is turned off.
4.3 Saw-tooth generator
The outputs of the saw-tooth generator are located
below the mounts at the rear of the tube and are
labelled -U and +U , respectively.
x x
A saw-tooth voltage (also frequently called a ramp) is
a voltage that changes periodically over time, increas-
ing or decreasing linearly from an initial value then
returning instantaneously to the start.
Caution: the saw-tooth voltage is relative to the anode
potential of +250 V.
The upper knob is used to turn on the generator and
make a coarse adjustment of the frequency. Fine
adjustment is completed with the lower knob.
5. Sample experiments
5.1 Electrical deflection of the electron beam
•
Set up the experiment as in fig. 2.
•
Turn off the voltage supply to the demonstration
oscilloscope
•
Connect deflecting plates to the output of the
saw-tooth generator.
•
Adjust the electron beam so that it strikes the
fluorescent screen on the left-hand side (about 1
cm from the edge).
•
Set the coarse adjustment of the saw-tooth fre-
quency to its minimum level (second position
from the left).
•
Turn on the voltage supply.
After 10-30 sec, the fluorescent dot appears on the
screen. It should migrate periodically from left to
right.
•
If necessary, decrease the frequency using the
fine adjustment knob so that the migration of the
point can be clearly tracked.
5.2 Magnetic deflection of the electron beam
•
Set up the experiment as in fig. 3.
•
Attach a coil to the metal ring.
•
Connect the inputs of the coil to the DC power
supply.
•
Adjust the electron beam so that it strikes the
centre of the fluorescent screen.
•
Turn on the DC power supply and vary the current
to the coil.
The beam is deflected perpendicularly to the direc-
tion of both the beam and the magnetic field.
•
Change the polarity and alignment of the coil and
the number of turns the current flows through
and observe the effects.
5.3 Trace of an AC voltage over time
Additional equipment required:
1 Function generator (50 Ω, with amplifier if possible)
or AC power supply.
Optional: 1 Multimeter with frequency counter
(maximum voltage, at least 150 V).
•
Set up the experiment as in fig. 4.
•
Follow the instructions for experiment 5.1, but do
not decrease the frequency, and set the coarse
adjustment to the medium level. If a multimeter
with frequency counter is available, connect it to
the outputs of the saw-tooth generator in parallel
with the deflection plates before turning on the
voltage supply. (Caution: it is dangerous to touch
the saw-tooth voltage outputs)
•
Attach a coil to the metal ring.
•
Connect the inputs of the coil to the function
generator (amplified if available).
•
Select a frequency between 30 and 100 Hz on the
function generator.
During movement from the left to the right side, the
beam is vertically deflected.
•
If necessary, increase output voltage to obtain a
bigger deflection.
Due to the rapid repetition, it is hard to see a wave-
form for the AC voltage, since recording usually does
not start the same point during the period (the same
phase) so that multiple phase-shifted images are
therefore superimposed. This problem does not occur
if the saw-tooth frequency is identical to the input
signal frequency of the function generator.
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