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Franck-Hertz Apparatus

You might recall from either your physics class or your chemistry class that atoms absorb energy, and in doing so they move electrons from rest states to higher orbits. These electron orbits are quantum in nature – either you have enough energy to go to a higher orbit or you don’t. Electrons do not go half-way to the next orbit. When an electron goes from a higher orbit to a lower one, it releases the energy of that orbit and the atom emits some sort of electromagnetic energy.

One of the earliest experiments demonstrating the electron orbits was the Franck-Hertz Experiment, where vacuum tube filled with heated Mercury vapor had electrons accelerated towards and anode grid with a slight negative charge. This negative charge kept the low-energy electrons away while registering the more energetic electrons.

As one might expect with the atomic model described above, the level of registered charge would rise and rise, then suddenly drop as the energy was enough to raise electrons in the mercury vapor to the next orbit. Then the charge would rise again, peak, and fall as before as the next orbit was reached, so on and so forth.


Duplicating the Franck-Hertz experiment has been a bit tricky, and that is because of the set-up involved with heated mercury, etc. To help physics teachers and professors duplicate the experiment we carry United Scientific’s Franck-Hertz Apparatus.


With this apparatus, a tube with argon vapor is used instead of heated mercury. The apparatus is shown hooked up to an oscilloscope (not included). The Oscilloscope is not required as direct readings can be taken from the apparatus from the dials on the device. Everything is internally contained int he device and it is compact and  easy to setup.  The current amplifier has four ranges  from 10^-5 to 10^-8 A for full scale deflection.  The tube voltage acceleration is adjustable from 0 to 100V.

So for one of the best experiments of showing how the Bohr model of the atom works, get yourself a Franck-hertz Apparatus!

Want to buy the Franck-Hertz Apparatus?

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