3B SCIENTIFIC PHYSICS U8482150 Instrucciones De Uso página 7

4. Operation
The following equipment is also required to com-
plete the experiment:
1 Control unit for Franck-Hertz experiment
1 Digital thermometer, single channel
1 Thermocouple NiCr-Ni, type K
1 Analog oscilloscope, 2 x 35 MHz
1 RF cable, 1 m
2 RF cable, BNC/4-mm plug
•
Turn off the heating chamber and the control
unit to begin with and connect terminals A, H
and K.
•
Connect terminal M of the Franck-Hertz tube to
the correct input on the control unit by means
of an RF cable.
•
Connect the "FH Signal y-out" terminal of the
control unit to the Y input of the oscilloscope
and terminal "UB/10 x-out" to the X input.
•
Immerse the NiCr-Ni thermocouple through
the opening at the top of the heating chamber
so that its tip just touches the Franck-Hertz
tube.
•
Turn all the knobs on the control fully to the
left.
•
Turn on the heating chamber. Set a tempera-
ture of about 130°C and wait for the tube to
warm up (about 5 to 10 minutes).
•
Set a filament voltage of 6 - 7 V. The indirectly
heated cathode requires about 90 seconds to
warm up, once the voltage is applied.
•
Turn on the control unit. Set the "Man/Ramp,
50 Hz" toggle switch to "Ramp" and slowly turn
up the accelerating voltage to 30 V.
•
Set up the oscilloscope in XY mode with
x = 0.5 V/div and y = 1 V/div.
•
Observe where the first maxima in the Franck-
Hertz trace on the oscilloscope screen occur.
•
Set up all the parameters, accelerating voltage,
cathode filament, bias voltage and amplitude
so that a trace with nicely delineated maxima
and minima is obtained.
•
Increase the filament voltage further and ob-
serve the emergence of more maxima.
•
Do not, however, increase the accelerating
voltage so much that self-discharge no longer
occurs inside the tube. Any ionisation due to
collisions will disrupt the curve.
The procedure as described so far is a general set-
ting procedure. Since the Franck-Hertz tubes are
hand-made, there may be quite large differences in
the optimum parameters from one tube to the
next. The test report included with the tube should
give some idea of where good results may be ob-
tained for the tube in question.
U8482130
The collector current displays regularly recurring,
equidistant maxima and minima that are inde-
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pendent of the accelerating voltage. The interval
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between these peaks is 4.9 V. A contact potential of
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2 V exists between the anode and cathode of the
tube, which is why the first maximum only appears
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in the region of 7 V.
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Evaluation of the Franck-Hertz curve:
To fully evaluate the Franck-Hertz curve, an X-Y
plotter and a digital voltmeter are needed. This
does not require that the current of the electron
beam be determined precisely. The oscilloscope
screen should show the trace of a Franck-Hertz
curve featuring very clear maxima and minima.
•
•
•
•
•
Note: The accelerating voltage is reduced by a
factor of 10 at the signal output (to the X-Y plotter).
The digital voltmeter, however, measures the full
accelerating voltage between sockets "A" and "K".
•
•
3
Set the "Man-Ramp/50 Hz" switch to "Man".
Turn the accelerating voltage knob all the way
to the left (U = 0 V).
B
Connect a digital voltmeter to sockets "A" and
"K" of the control unit to measure the acceler-
ating voltage.
Connect up an X-Y plotter instead of the oscil-
loscope.
Turn on the X-Y plotter and set both axes to a
gain of 1 V/cm.
Vary the gain of the plotter so that a Franck-
Hertz curve is obtained that covers the full
width of the paper.
By gradually increasing the accelerating volt-
age U at a constant rate, a Franck-Hertz curve
B
can be recorded and the precise position of the
maxima and minima determined with the aid
of the digital voltmeter.