3B SCIENTIFIC PHYSICS 1012819 Instrucciones página 9

4. Examples of use
4.1 Franck-Hertz tube with Hg filling
Additionally required:
1 F/H tube w. Hg filling a. heating chamber (230 V,
50/60 Hz) 1006795
or
1 F/H tube w. Hg filling a. heating chamber (115 V,
50/60 Hz) 1006794
1 Analogue oscilloscope, 2x 30 MHz 1002727
1 HF patch cord, 1 m
2 HF patch cords, BNC / 4-mm plug
Safety leads for experiments

Place front plate of the open side of the heat-
ing chamber and fix it in place with 6 knurled
screws.

Turn off the heating chamber and the control
unit to begin with and turn all the knobs on the
control fully to the left.

Do not apply a voltage to the tube when it is
still cold (the mercury inside may cause a
short circuit).

Connect terminals "A", "F" and "K" (refer to
Fig. 1).

Connect terminal "E" of the Franck-Hertz tube
to the correct input on the control unit by
means of an BNC cable.

Connect the "FH Signal UY-out" terminal of
the control unit to the Y input of the oscillo-
scope and terminal "UX" to the X input.

Turn on the heating chamber. Set a tempera-
ture of about 210° C and wait for the tube to
warm up (about 5 to 10 minutes).

Turn on the control unit and the equipment
should enter ramp mode.

Set a filament voltage of 6 − 7 V. The indirectly
heated cathode requires about 90 seconds to
warm up, once the voltage is applied.

Set the minimum acceleration voltage to zero,
slowly increase the maximum acceleration
voltage to 80 V.

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.

Set up the oscilloscope initially with settings of
x = 1 V/div and y = 1 V/div.

Observe the emergence of the maxima in the
Franck-Hertz trace
screen.
1002746
1002748
on the oscilloscope

Set up all the parameters, accelerating volt-
age, cathode filament, bias voltage and ampli-
tude so that a trace with nicely delineated
maxima and minima is obtained.
The procedure as described so far is a general
setting procedure. Since the Franck-Hertz tubes
are hand-made, there may be quite large differ-
ences 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 obtained for the tube in question.
The collector current displays regularly recurring,
equidistant maxima and minima that are independ-
ent of the accelerating voltage. The interval be-
tween 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 tempera-
ture.

Set the minimum acceleration voltage to zero,
choose a maximum acceleration voltage 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 cathode
and the grid. Then turn down the filament volt-
age till the glow disappears and only the fila-
ment is glowing.
3
1000912
1002727
1002746
1002748