2
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
1 Power switch
2 Operating display
3 “SET” button
4 Display
5 “+/-” buttons
6 Thermal insulation
7 Knurled screws
8 Front plate with Franck-
8
Hertz tube attached (not
8
shown)
9 Cathode socket
10 Tube filament socket
11 Anode socket
12 BNC socket for signal
12
output
13 Heating chamber
14 Spring clip for thermom-
eter
15 Handle
