You Too Can Do The Franck-Hertz Experiment

We talk about quantum states — that is, something can be at one of several discrete values but not in between. For example, a binary digit can be a 1 or a 0, but not 0.3 or 0.5. Atoms have quantum states, but how do we know that? That’s what the Franck-Hertz experiment demonstrates, and [stoppi] shows you how to replicate that famous experiment yourself.

You might need to translate the web page if your German isn’t up to speed, but there’s also a video you can watch below. The basic idea is simple. A gas-filled tube sees a large voltage across the cathode and grid. A smaller voltage connects to the grid and anode. If you increase the grid voltage, you might expect the anode current to increase linearly. However, that doesn’t happen. Instead, you’ll observe dips in the anode current.

When electrons reach a certain energy they excite the gas in the tube. This robs them of the energy they need to overcome the grid/anode voltage, which explains the dips. As the energy increases, the current will again start to rise until it manages to excite the gas to the next quantum level, at which point another dip will occur.

Why not build a whole lab? Quantum stuff, at a certain level, is weird, but this experiment seems understandable enough.

One thought on “You Too Can Do The Franck-Hertz Experiment

  1. So, the original experiment used mercury vapour to fill the tube, and that is the gas that is analyzed. This version seems to depend on the xenon that’s present in this particular type of tube (a thyratron).

    I’m guessing it won’t work with just any old vacuum tube.

    Would be interesting to know how do you distinguish this effect from similar low-voltage non-linear space charge effects in vacuum tubes.

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