Franck–Hertz Apparatus – Neon Tube with Heater

The Franck–Hertz tube is a tetrode with an indirectly heated barium-oxide cathode, a mesh-type control grid, a mesh-type anode and a collector electrode in a plane-parallel arrangement; the control grid sits about 5 mm from the anode grid, while the cathode-to-grid and anode-to-collector spacings are each about 2 mm. The tube is supplied pre-filled with neon at a pressure chosen to give an optimal characteristic curve, and the connection sockets for the heater, control-grid and anode-grid voltages are on the base, with the collector current taken from the BNC socket at the top. An internal 10 kΩ limiting resistor is permanently built in between the accelerator (control-grid) and anode voltages to protect the tube against a spark discharge caused by excessive voltage. It is used in atomic-physics teaching to demonstrate the quantisation of energy and electron collision processes, with pupils varying the accelerating voltage and observing the characteristic Franck–Hertz curves that reveal the energy levels of neon atoms.