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15 × 3.5 × 3.5 cm

This tube is specially made for repeating the experiment that won James Franck and Gustav Hertz the 1925 Nobel Prize for Physics.

 The experiment demonstrated the existence of quantised excited states by showing that electrons accelerated to  particular energies were more effective in ionising mercury vapour.

The tube has a triode structure similar to an ionisation gauge, with a heated cathode to emit electrons, a grid positively biased to accelerate them to a known energy, and a third, negatively biased electrode to collect the mercury ions produced by collisions.