3B SCIENTIFIC PHYSICS U8482230 Instructions D'utilisation page 6

accelerator (control grid) voltage and the anode volt-
age. This protects the tube in case there is a spark
discharge caused by applying too high a voltage. The
voltage loss in this resistor when making measure-
ments is negligible, as the anode current in the tube
is smaller than 5 pA. (Thus the voltage loss in the
protecting resistor is 0.05 V.)
3. Technical data
Filament voltage:
Control voltage:
Accelerating voltage:
Countervoltage:
Tube:
Base with connector
sockets:
Weight:
4. Basic principles
In the Franck-Hertz experiment neon atoms are ex-
cited by inelastic collision with electrons. The excited
atoms emit visible light that can be viewed directly.
Thus it is possible to detect zones where the light and
therefore the excitation is more intense. The distribu-
tion of such zones between the cathode and the grid
depends on the difference in potential between the
two:
Electrons are emitted from the cathode and are accel-
erated by a voltage U towards the grid. Having passed
through the grid they reach the target and thus con-
tribute to a target current I if their kinetic energy is
sufficient to overcome a decelerating voltage between
the grid and the target.
The I(U)-characteristic (see Fig. 3) has a similar pattern
to the original Franck-Hertz experiment using mer-
cury gas but this time the intervals between minima
where the current falls to almost zero for a specific
voltage U = U corresponding to the electrons reach-
1
ing sufficient kinetic energy to excite a neon atom by
inelastic collision just before reaching the grid are
about 19 V. Simultaneously it is possible to observe a
faint orange light close to the grid since the energy
transition to the base state of a neon atom results in
the emission of such light. The zone of illumination
moves towards the cathode as the voltage U increases
and the target current I rises once more.
For a higher voltage U = U
drops drastically and it is possible to see two zones of
illumination. The electrons can in this case retain
enough energy after an initial collision to excite a
second neon atom.
4 − 12 V
9 V
max. 80 V
1.2 − 10 V
130 x 26 mm dia. approx.
3
190x115x115 mm approx.
450 g approx.
the target current also
2
As the voltages are further increased, other minima in
the target current along with further zones of illumi-
nation can be observed.
The I(U)-characteristic exhibits various maxima and
minima and the interval between the minima is
about ΔU = 19 V. This corresponds to excitation en-
ergy of the 3p energy level of a neon atom (see Fig. 4)
so that it is highly likely that this level is being ex-
cited. Excitement of the 3s-level cannot be neglected
entirely and gives rise to some fine detail in the struc-
ture of the I(U)-characteristic.
The zones of illumination are zones of greater excita-
tion and correspond to drops in voltage in the I(U)-
characteristic. One more zone of illumination is cre-
ated every time U is increased by about 19 V.
Note
The first minimum is not at 19 V itself but is shifted
by an amount corresponding to the so-called contact
voltage between the cathode and grid.
The emission lines in the neon spectrum can easily be
observed and measured using a spectroscope
(1003184 / U21877) when the maximum voltage U is
used.
5. Operation
The following equipment is also required to complete
the experiment:
1 Power supply unit for F/H experiment (230 V, 50/60 Hz)
or
1 Power supply unit for F/H experiment (115 V, 50/60 Hz)
1 Analogue oscilloscope, 2x 30 MHz 1002727 / U11175
1 HF Patch cord, 1 m
2 HF Patch cords, BNC/4 mm plug 1002748 / U11257
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 temperature.
•
Set the accelerating voltage to 80 V and select a
grid voltage of 9 V.
2
1012819
1012818
1002746 / U11255