4
Press the “Man/Ramp” button and the dis-
play 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 be-
tween sockets "A" and "K" in order to obtain a
more accurate measure of the accelerating volt-
age.
Note:
The accelerating voltage is reduced by a
factor of 10 at the signal output (
U
X
). The digital
voltmeter, however, measures the full acceler-
ating 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.
Curve optimization
Fig. 5: Illustration of various signals of the
Franck-Hertz tube.
The parameters of the tube can change during
the use of the tube. This usually happens either
due to ageing of the tube or due to the experi-
mental specification. Figure
5.f)
represents the
ideal example of the curve. However, other
types of curves can also occur.
Breakdown of the tube:
Figure
5.a)
shows the case of a breakdown of
the tube. The anode current increases rapidly to
a maximum value. In this case, the accelerating
voltage must be lowered immediately. If it
should be necessary to map further maxima,
the temperature of the furnace must be in-
creased beforehand.
Optimization of the countervoltage:
Figures
5.b)
and
5.c)
show a too steep or too
flat curve with few maxima. The countervoltage
determines the slope of the curve. The greater
the countervoltage, the flatter the rise. In com-
bination with the acceleration voltage, the qual-
ity of the maxima can be slightly improved.
Optimization of the filament voltage:
The filament voltage determines the number of
emitted electrons and thus the anode current.
The higher the filament voltage, the more elec-
trons are emitted. Figure 5.d) shows the case
where the anode current is too high. The signal
flattens out to a horizontal line above a thresh-
old value. The maximum number of available
electrons is reached (under the given filament
voltage) and the anode current remains con-
stant, despite the increasing accelerating volt-
age. Figure 5.e) shows the case of a too low fil-
ament voltage. The signal has a low rise and the
maxima are weak, although there is a suffi-
ciently high number of maxima. In these cases,
lowering or increasing the filament voltage is of-
ten sufficient to obtain an evaluable curve. Note:
An excessively high filament voltage has a neg-
ative effect on the life of the tube. It is not rec-
ommended to operate the tube with too high fil-
ament voltages.
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 lo-
cal recycling points.
Should you need to dispose of the equip-
ment itself, 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 dis-
posal of mercury must be
followed.
