3B Scientific Premium Franck-Hertz Experiment Heater (230 V, 50__60 Hz) - Manual (Page 3)

3
4. Operation
The following equipment is also required to com-
plete the experiment:
1 Power supply unit for F/H experiment (230 V, 50/60 Hz)
1012819
or
1 Power supply unit for F/H experiment (115 V, 50/60 Hz)
1012818
1 Analogue oscilloscope, 2x 30 MHz 1002727
1 HF Patch cord, 1 m 1002746
2 HF Patch cords, BNC/4 mm plug 1002748
Safety leads for experiments
• Place front plate of the open side of the heating
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 cir-
cuit).
• Connect terminals "A", "F" and "K" (refer to fig. 2).
• 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 U
Y
-out” terminal of the
control unit to the Y input of the oscilloscope
and terminal “U
X
” to the X input.
• Turn on the heating chamber. Set a temperature
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 volt-
age to 80 V.
• Do not, however, increase the accelerating volt-
age 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 on the oscilloscope screen.
• 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.
The procedure as described so far is a general setting
procedure. Unavoidable differences resulting from
the manufacture of individual Frank-Hertz tubes
mean that the optimum parameters may differ from
tube to tube. 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 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.
Evaluation of the Franck-Hertz curve:
To fully evaluate the Franck-Hertz curve, a digital
voltmeter is needed. This does not require that the
current of the electron beam be determined pre-
cisely. The oscilloscope screen should show the trace
of a Franck-Hertz curve featuring very clear maxima
and minima.
• Connect a digital voltmeter between the signal
output (U
X
) and the ground socket (refer to fig. 3).
• Press the “Man/Ramp” button and the display
will show “Man” to indicate manual mode.
• Turn the accelerating voltage knob all the way to
the left (U
A
= 0 V).
The display will show the accelerating voltages in
steps of 0.5 V. In order to obtain better test results, a
digital voltmeter can be connected between sockets
"A" and "K" in order to obtain a more accurate
measure of the accelerating voltage.
Note: The accelerating voltage is reduced by a factor
of 10 at the signal output (U
X
). The digital voltmeter,
however, measures the full accelerating voltage
between sockets “A” and “K”.
By gradually increasing the accelerating voltage at a
constant rate, the precise position of the maxima
and minima can be determined with the aid of the
digital voltmeter.
5. Care and maintenance
• Before cleaning the equipment, disconnect it
from its power supply.
• Use a soft, damp cloth to clean it.
6. Disposal
• The packaging should be
disposed of at local recy-
cling points.
• Should you need to dis-
pose of the equipment it-
self, never throw it away
in normal domestic waste.
Local regulations for the
disposal of electrical
equipment will apply.
• If Frank-Hertz tubes are to
be disposed of, local regu-
lations applying to the
disposal of mercury must
be followed.