2
Only trained electricians are permitted to
open up the apparatus’ housing.
Beware: Risk of burns! The viewing windows and
the walls of the heating chamber can reach tem-
peratures of up to 300° C during operation.
Set up the heating chamber on a heat-re-
sistant surface.
When the heating chamber is in operation, do
not attempt to touch or move the apparatus.
Only move or transport the equipment by us-
ing the insulated handle.
Allow the apparatus to cool before disman-
tling the experiment.
Beware: There is always a risk that glass can
break and cause injury.
Use all six knurled screws to affix the front
plate to the heating chamber.
Do not subject the tube to any mechanical
stress. Do not put kinks in any connecting
leads.
The Franck-Hertz tube contains mercury.
If the glass breaks and mercury escapes,
make sure to observe safety regulations re-
lating to the handling of mercury.
2. Description
The Franck-Hertz tube with its mercury filling is
used to demonstrate the quantised release of en-
ergy by free electrons colliding with mercury at-
oms and to determine that the excitation energy
for the mercury resonance line (6
1
S
0
– 6
3
P
1
) is 4.9
eV.
Franck-Hertz tube attached to front plate
The Franck-Hertz tube is a highly evacuated elec-
tron tube containing mercury with its electrodes set
up in parallel planes. The electrodes consist of an
indirectly heated oxide cathode, an anode grid and
a collector electrode. In order to increase the like-
lihood of collisions, the distance between the an-
ode and cathode has been made large (8 mm) in
comparison to the mean free path in a mercury at-
mosphere (for a temperature of approx. 180°C).
By contrast, the distance between anode and col-
lector is small. An earth ring is located level with
the anode grille to prevent disturbance due to
background radiation. The tube is mounted on the
front plate of the heating chamber and can be re-
moved or exchanged. The front plate also has ce-
ramic-insulated sockets and a schematic diagram
of the tube. The Franck-Hertz tube is mounted in
such a way that the whole tube and its connecting
wires can all be maintained at constant tempera-
ture. This is necessary because the density of the
mercury vapour is always determined at the cold-
est point of the tube. Leakage currents resulting
from radiation due to ion conduction in the hot
glass walls are prevented by a protective sintered
alumina ring. A fixed attenuating resistor (10
is
inserted between the sockets for the accelerating
voltage and for the anode of the tube. This protects
the tube in the event of excessive voltage occur-
ring due to arcing. The voltage drop across this re-
sistor can be neglected when measurements are
being made.
Heating chamber
The heating chamber serves to establish the va-
pour pressure inside the Franck-Hertz tube with its
mercury filling and helps you carry out experiments
with a sodium fluorescence tube (1000913).
It consists of a powder-coated sheet steel casing
with two viewing windows. The front plate is at-
tached via six knurled screws. The chamber is
heated via a tubular heating element in the cham-
ber floor. Temperature measurement and regula-
tion is carried via an integrated micro-controller
and a PT 100 thermocouple. A digital tempera-
ture display allows you to read off the tempera-
ture set-point and the actual temperature value.
The “SET” button can be used to toggle the dis-
play between ° Celsius and ° Fahrenheit. The
“+/-
” keys allow you to set the set-point for the tem-
perature in steps of 1 K. There is an opening at
the top with a spring clip for holding a thermometer
and a thermally insulated carrying handle.
The apparatus 1023094 is for operation with a
mains voltage of 115 V (±10%), and the unit
1023095 is for operation with a mains voltage of
230 V (±10%).
2.1 Scope of delivery
1 Franck-Hertz tube with mercury filling mounted
on front-plate
1 Heating chamber without front plate
1 Instruction sheet
3. Technical data
Franck-Hertz tube
Filament:
4 to 9 V AC/DC
Grid voltage:
0 to 80 V
Bias voltage:
1.5 V approx.
Operating temperature: 160°C - 200° C approx.
Tube dimensions:
160 mm x 30 mm diam.
Weight:
380 g approx.
