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 level laboratory. 

Argon filled tetrode

Filament Power Supply : 2.6-3.4V continuously variable

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

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.

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.