From the early spectroscopic
work it is clear that atoms emitted radiation at discrete
frequencies; from Bohr’s model, the frequency of the
radiation n is related to the change of energy levels through
DE=hn. It is then to be expected that transfer of energy to
atomic electrons by any mechanism should always be in discrete
amounts. One such mechanism of energy transfer is through
inelastic scattering of low-energy electrons.
Frank and Hertz in 1914 set out to verify these considerations.
- It is possible to excite atoms by low
energy electron bombardment.
- The energy trasferred from electrons
to the atoms always had discrete values.
- The values so obtained for the energy
levels were in agreement with spectroscopic results.
Thus the existence of atomic energy levels
put forward by Bohr can be proved directly. It is a very important
experiment and can be performed in any college or University
Frank-Hertz tube in this instrument is a tetrode filled with
the vapour of the experimental substance. Fig.1 indicates
the basic scheme of experiment. The electrons emitted by filament
can be accelarated by the potential VG2K
between the cathode and the grid GV2. The grid
G2 helps in minimising space charge effects. The
grids are wire mesh and allow the electrons to pass through.
The plate A is maintained at a potential slightly negative
with respect to the grid V2 GV2. This
helps in making the dips in the plate current more prominent.
In this experiment, the electron current is measured as a
function of the voltage VG2K. As the
voltage increases, the electron energy goes up and so the
electron can overcome the retarding potential VG2A
to reach the plate A. This gives rise to a current in the
ammeter, which initially increases. As the
voltage further increases, the electron energy reaches the
threshold value to excite the atom in its first allowed excited
state. In doing so, the electrons loose energy and therefore
the number of electrons reaching the plate decreases. This
decrease is proportional to the number of inelastic collisions
that have occured.When the VG2K is
increased further and reaches a value twice that of the first
excitation potential, it is possible for an electron to excite
an atom halfway between the grids, loose all its energy, and
then gain anew enough energy to excite a second dip in the
current. The advantage of this type of configuration of the
potential is that the current dips are much more pronounced,
and it is easy to obtain five fold or even larger multiplicity
in the excitation of the first level.
Frank-Hertz Experiment Set-up, Model: FH-3001, consists
of the following:
- Argon filled tetrode
- Filament Power Supply : 2.6-3.4V continuously
- Power Supply for VG1K
: 1.3-5V continuously variable
- Power Supply for VG2A
: 1.3 - 12V continuously variable
- Power Supply for VG2K
: 0 - 95V continuously variable
All the power supplies are highly stabilised and output voltages can be read on 31/2 digit, 7 segment LED DPM with autopolarity and decimal indication through a selector switch.
- Saw tooth waveform for CRO display
- Scanning Voltage : 0-80V
- Scanning Frequency : 115±20Hz
- Multirange Analogue Voltmeter
- Range : 0-5V, 0-15V & 0-100V
- Multirange Digital Ammeter
- Display : 31/2 digit 7 segment LED
- Range Multiplier : 10-7, 10-8, 10-9
- Power : 110V±10% mains, 50Hz.
The instrument can, not only lead to a plot
of the amplitude spectrum curve by means of point by point
measurement, but also directly display the amplitude spectrum
curve on the oscilloscope screen. This instrument can thus
be used as a classroom experiment as well as for demonstration
to a group of students.
Analysis of Data
Data obtained for the excitation potential
point by point are shown in Fig. 3. The readings are taken
for 1V changes on grid 2 (VG2K). A significant
decrease in electron (collector) current is noticed every
time the potential on grid 2 is increased by approximately
12V, thereby indicating that energy is transferred from the
beam in (bundles) "quanta" of 12eV only. Indeed,
a prominent line in the spectrum of argon exists at 1048Å
corresponding to eV=11.83.
The location of the peaks is indicated in
Fig. 3. Average value of spacing between peaks is 11.75eV compared
with the accepted value of 11.83V.