3B SCIENTIFIC PHYSICS U8481350 Instructions D'utilisation page 10

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With the fine adjustment, search for a frequency
at which a standing image appears, which shows
a clear period of oscillation.
At which other saw-tooth frequencies does a standing
image also appear?
5.4 Lissajous figures
Additional equipment required:
1 Function generator (50 Ω, with amplifier if possi-
ble), and 1 AC power supply or 2 function generators.
•
Set up the experiment as in fig. 5.
•
Attach a coil to the metal ring pointing inward,
with the axis aligned horizontally.
•
Connect the inputs (green, yellow) to the AC
power supply or to the second function generator
(set to a 50 Hz sinusoidal voltage). Select the am-
plitude so that the line appearing on the screen is
approximately half the length of the screen di-
ameter.
•
With the circular magnet, adjust the line horizon-
tally to centre it.
•
Mount another coil to the metal ring pointing
inward, with the axis aligned vertically.
•
Connect the inputs (green, yellow) to the first
function generator (set to a 50 Hz sinusoidal volt-
age).
Fig.2 Electrical deflection of the electron beam (left: with power supply U8521371, right: with power supply U33000)
An ellipse appears that changes shape at faster or
slower speed, depending on how well the frequencies
of the input signals match. This takes the shape of a
sloped straight line twice per cycle.
•
Adjust the amplitude of the first function genera-
tor so that the slope of the straight line is 45° and
that a circle emerges during transition.
The simplest Lissajous figures can be observed al-
ready. The shapes depend on the frequency ratios and
on the phase shift. Due to a small deviation from the
exact target frequency on either of the two function
generators (usually, the inaccuracy of the devices is
already sufficient), the phase shift cycles automati-
cally, and all figures for a specific frequency ratio can
be observed in succession.
•
Set the frequency of the first function generator
to a multiple of the horizontal frequency (50 Hz).
Observe Lissajous figures for the frequency ratios 2:1,
3:1 and 4:1.
•
Further Lissajous figures are created by fractional
multiples of the horizontal frequency (e.g., 3:2 (75
Hz), 4:3 (66.7 Hz).
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