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Franck Hertz Experiment


Model: HO-ED-EM-04

The Franck - Hertz Apparatus (Model No: HO-ED-EM-04) is designed for verifying the existence of quantized states. 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.

The Franck-Hertz experiment is a fundamental quantum physics experiment which confirmed the quantization of atomic energy levels. This experiment supports Bohr model of atom. Apparatus used for the experiment consist of a tube containing low pressure gas, fitted with four electrodes.

The four electrodes are: an indirectly heated, oxide coated, cathode as an electron source, two grids G and G and a plate A, which serves as an 1 2 electron collector. In this experiment, the electron beam is produced by thermionic emission from a filament. The electrons are accelerated, passed through the vapour, and are then retarded (decelerated) by a few volts before collection at the anode. All these takes place in an argon filled tube.

When an electron has an inelastic collision with an argon atom, the kinetic energy lost to the atom causes one of the outer orbital electrons to be pushed up to the next higher energy level. This excited electron, within a very short time, falls back into the ground state level, emitting energy in the form of photons. The original bombarding electron is again accelerated toward the grid anode and this excitation energy can be measured.

In the spectrum amplitude curve , the voltage difference between two consecutive peak point is the first excitation potential of argon atom. This experiment illustrates the fact that the electrons in the Franck ‐ Hertz tube collide with argon atoms and excite the atoms from low level to high level. By measuring the argon's first excitation potential we can verify that the energy absorbed and transmitted is discrete, not continuous.

Experiment Examples

    To measure the excitation potential of Argon using the Franck - Hertz method

e/m   =   [  (  125 a2  /  128 π2 N2  )  x  1014  ]  (  V  /  I2 D2  )

    To verify that atomic systems have discrete energy levels by bombarding electrons and observing the difference in energy levels

e/m   =   [  (  125 a2  /  128 π2 N2  )  x  1014  ]  (  V  /  I2 D2  )


    Franck - Hertz Tube   :   Argon Filled Tetrode

Power Supply Unit

    VG1K   :   1.20 – 5.00 V (continuously variable)

    VG2K   :   0.00 – 95.00 V (continuously variable)

    VG2A   :   1.30 - 14.50 V (continuously variable)

    Filament Voltage   :   2.8 - 3.40V (continuously variable)

Saw tooth waveform for CRO display

    Scanning Voltage   :   0 - 90 V

    Scanning Frequency   :   18 ± 2 Hz

    Input Power   :   230 V / 50 Hz

Fig: Current Measurements in the Frank-Hertz Experiment

Related Topics

    Quantum energy

    Electron collision

    Excitation energy

    Inelastic collision

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