Physics for CSS

On 24 April in 1914, the Franck-Hertz experiment was first presented in a paper in the German Physical Society by James Franck and Gustav Hertz.

Their experimental results supported a theory previously put forward by Niels Bohr – which stated that atoms were made up of a nucleus orbited by electrons at fixed energies.

In the experiment Franck and Hertz applied a potential difference to a tube containing a low-pressure gas. When increased, the current flowing through the tube also increased until the voltage reached a certain level, when it suddenly declined.

Even though they hadn't heard of Bohr's atomic model – based on early quantum theory – when they performed their experiment, Franck and Hertz later realised their paper supported it. The potential difference in their experiment increased the free electrons' mobility until the electrons had enough energy to jump up to the next energy level, just like Bohr predicted.

Photos left to right: James Franck, a vacuum tube used in the experiment (E. A. Schiff Wikimedia commons) and Gustav Hertz.

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