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The collector current displays regularly recurring,
equidistant maxima and minima that are inde-
pendent of the accelerating voltage. The interval
between these peaks is 4.9 V. A contact potential
of 2 V exists between the anode and cathode of
the tube, which is why the first maximum only
appears in the region of 7 V. The first maxima will
be more obvious when the temperature of the
heating chamber is lower.
4.2 Franck-Hertz tube with Ne filling
Additionally required:
1 Franck-Hertz tube with Ne filling
1 Analogue oscilloscope, 2x 30 MHz
1 HF lead, 1 m
2 HF leads, BNC / 4-mm plug
Safety leads for experiments
•
Start with the voltage supply unit switched
off, and with all the voltage setting knobs fully
to the left.
•
Connect up the experiment as shown in Fig.
2.
•
Turn on the equipment. It will start in ramp
mode.
•
Set up the oscilloscope in XY mode with
x = 1 V/div and y = 2 V/div.
•
Gradually increase the heater voltage till the
filament starts to faintly glow red. Then wait
30 seconds till it reaches its operating tem-
perature.
•
Set the minimum acceleration voltage to
zero, choose a maximum acceleration volt-
age of 80 V and set the control grid voltage
to 9 V.
The ideal filament voltage should be between 4
and 12 V. This differs from tube to tube due to
manufacturing tolerances.
•
Gradually increase the filament voltage until
an orange glow appears between the cath-
ode and the grid. Then turn down the filament
voltage till the glow disappears and only the
filament is glowing.
•
Gradually increase the decelerating voltage
until the measured curve (of signal against
accelerating voltage) is near horizontal.
•
Increase the gain till the maxima of the
Franck-Hertz curve can be seen on the oscil-
loscope screen.
4.3 Critical potential tube
Additionally required:
1 Critical potential tube S with He-filling
or
1 Critical potential tube S with Ne-filling
1 Tube holder S
1 Analogue oscilloscope, 2x 30 MHz
1 HF lead, 1 m
2 HF leads, BNC / 4-mm plug
Safety leads for experiments
Observation of
potentials
•
Fit the critical potential tube into the tube
holder, ensuring that the contact pins of the
tube are correctly and fully engaged in the
tube socket of the holder. The central guide
pin of the tube should project slightly at the
back of the holder.
•
Start with the voltage supply unit switched
off, and with all the voltage setting knobs fully
to the left.
•
Connect terminal F3 on the tube holder with
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output F of the control unit, connect C5 to
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output K (C5 and F4 are connected together
internally inside the tube) and A1 to output A
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(refer to Fig. 3).
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•
Fit the screening frame over the tube, push
its folded edge into the opening in the tube
holder and connect it to the earth socket on
the control unit.
•
Connect the lead from the collector ring to
the F/H signal input socket E.
•
Connect the "FH Signal U
the control unit to the Y input of the oscillo-
scope and terminal "U
•
Set up the oscilloscope initially with settings
of x = 1 V/div and y = 1 V/div.
•
Set the minimum acceleration voltage to
about 15 V and the maximum acceleration
voltage to about 28 V.
•
Set a filament voltage of 2.7 V.
•
Increase the filament voltage slightly and
optimize the minimum and maximum accel-
eration voltage U
•
In the spectrum, identify the 2
19.8 eV and determine its position t
time axis.
•
Identify the ionisation threshold at 24.6 eV.
Observation of ionisation
•
In order to observe ionisation change the
sign for the polarity of the counter voltage.
•
Keep the apparatus in a clean, dry and dust
free place.
•
Before cleaning the equipment, disconnect it
from its power supply.
•
Do not clean the unit with volatile solvents or
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abrasive cleaners.
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•
Use a soft, damp cloth to clean it.
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The packaging should be disposed of at local
•
recycling points.
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•
Should you need to dispose of the equipment
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itself, never throw it away in normal domestic
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waste. Local regulations for the disposal of
electrical equipment will apply.
3
" to the X input.
X
.
A
5. Storage, cleaning, disposal
critical
-out" terminal of
Y
3
S peak at
on the
1
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