Verification of Bohar’s Atomic orbital theory by Frank-Hertz experiment


From Bohr’s postulation we know that the internal energies of an atom are quantized. We can directly prove this postulation by this experiment. In this experimental set up there is tetrode tube filled with argon vapor. Electrons emitted by the heated filament are accelerated by the potential VG2K (applied between cathode and grid2). At first ,some electrons reached to the plate A provided that their kinetic energy is sufficient to overcome the retarding potential VG1K(applied between cathode and grid1). So initially we see that the plate current increases with increase of  VG2K . But as the voltage further increases the electron energy reaches the threshold value to excite the atom in its first exited state, the abruptly drops. When the VG2K is increased further again the current starts to increase  and when VG2K reaches to a value twice that of first excitation potential again we get the current  drops. This way we get that the current drops 4-5 times at a certain voltage range. This observation proves that the energies of the atom are quantized.

Experimental results:

VG2K (Volt)Plate current (nA)



A significant decrease in electron current is noticed every time the potential on grid 2 is increased by approximately 12 volt, there by indicating that energy is transferred from the beam quanta 12 eV only.

According to our experiment the average spacing of peaks comes out as …….eV.

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