Franck Hertz Experiment – Mercury

Observe the spectral lines of mercury atoms.

The mercury atoms are enclosed in a tetrode with cathode, grid-type control electrode, acceleration grid and target electrode. The control grid ensures a virtually constant emission current of the cathode. An opposing voltage is applied between the acceleration grid and the target electrode. When the acceleration voltage U between the cathode and the acceleration grid is increased, the target current I corresponds closely to the tube characteristic once it rises above the opposing voltage. As soon as the electrons acquire sufficient kinetic energy to excite the mercury atoms through inelastic collisions, the electrons can no longer reach the target, and the target current drops. At this acceleration voltage, the excitation zone is directly in front of the excitation grid. When the acceleration voltage is increased further, the excitation zone moves toward the cathode, the electrons can again accumulate energy on their way to the grid and the target current again increases. Finally, the electrons can excite the mercury atoms once more, the target current drops again, and so forth. The I(U) characteristic thus demonstrates periodic variations, whereby the distance between the minima ΔU = 4.9 V corresponds to the excitation energy of the mercury atoms from the ground state to the first state.

Hg Franck-Hertz tube
Socket for Hg Franck-Hertz tube, with DIN connector
Electric oven for 230 V
Franck-Hertz operating device
Temperature probe NiCr-Ni, 1.5 mm
Oscilloscope 30 MHz, two-channel, analogous
Screened cable BNC/4 mm plug (2)