Experiment: Franck-Hertz Experiment for Mercury
(115 V, 50/60 Hz)

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Description
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Thematic focus:_..

• Record and evaluate the Franck-Hertz curve for mercury.
• Determining the separation ΔU of current maxima or minima.
• Compare the voltage intervals with the excitation energies of mercury atoms.

Experiment description:_..

The Franck-Hertz experiment for mercury involves observing how energy is transferred from electrons as a result of inelastic collision while passing through mercury vapour. The transfer of energy occurs in discrete steps corresponding to the excitement by such collision of distinct energy level transitions in the mercury atoms. The experiment thus provides confirmation of the Bohr model of the atom and the discrete energy levels described by that model.

Observation and Interpretation:_..

An evacuated glass tube contains a heated cathode C, a grid G and a target electrode A placed in that sequence. Electrons are emitted from the cathode and are accelerated by a voltage U towards the grid. Having passed through the grid they reach the target and thus contribute to a target current I if their kinetic energy is sufficient to overcome a decelerating voltage U_GA between the grid and the target. In addition a glass tube with a droplet of mercury is included and this is heated to generate a vapour pressure of approximately 15 hPa.
As the voltage U increases the target current I initially increases since more and more atoms are attracted out of the space charge field around the cathode by the electric field. At a certain value U = U₁ some atoms attain sufficient kinetic energy just in front of the grid so that they are able to provide sufficient energy to excite the mercury atoms by inelastic collision. The target current then drops to near zero since after such a collision, the electrons no longer have the energy to overcome the decelerating voltage.
As the voltage increases more, the electrons acquire enough energy to excite the mercury atoms further away from the grid. After such collisions they are accelerated again and can once again acquire enough energy to reach the target so the target current rises again.
At a still higher voltage U = U₂ the electrons can acquire so much energy after the first collision that they are able to excite another mercury atom. The target current once again drops drastically but rises once more as the voltage further increases. This continues for a third time at a still higher voltage and again the target current drops dramatically.