Franck-Hertz Mercury System

Franck-Hertz Mercury System



This system includes the Franck-Hertz tube with mercury filling and Heater, as well as the Power Supply Unit. This is a recreation of the experiment that provided some of the first evidence in support of the Bohr model of the atom. Electrons can transfer only quantized amounts of energy to the atoms, resulting in a current that varies periodically as the accelerating voltage increases. This experiment with a Mercury tube is most like the original experiment performed by James Franck and Gustav Hertz in 1914.

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In 1913 Franck and Hertz first performed the important experiment demonstrating discrete energy states in atoms. This experiment demonstrating the quantization of energy as well as the recording and evaluation of spectra is included in most of the curricula used around the world. 

This experiment can be performed with the Franck-Hertz tube described below. There are two different tubes available, a Neon filled tube and a Mercury filled tube (which requires a heater). Both tubes require the specially designed power supply shown as well as an oscilloscope (not included). We have packaged the required items in convenient systems, however, replacement tubes are available separately. Additionally if you want to purchase both the Ne and Hg tube only one of the special power supplies is required. 

Franck-Hertz Tube with Mercury and Oven
Free electrons colliding with mercury atoms emit energy in quantized packet. The excitation energy at 4.9 eV can be determined. A highly-evacuated cathode ray tube with mercury gas filling and plane-parallel electrodes in order to avoid distortion of the field lines. Electrodes include an indirectly heated, oxide cathode, perforated to produce a homogenous electric field plus an anode grid and collector (counter) electrode. In order to raise the probability of collisions between electrons and mercury atoms to a sufficient amount, the tube needs to be heated using a special heater. This heater is used for adjusting the vapor pressure in the mercury filed tube. It is a spray painted metal housing with the tube symbols printed on the front panel that are visible from a distance, ceramically insulated connection sockets, viewing windows, opening with spring clamp for thermometer in the lid, insulated carrying handle and built-in thermostat for adjusting temperature. The system includes the Franck-Hertz mercury tube with oven and the Franck-Hertz power supply. 



Heater 4-12 V AC/DC

Grid voltage 0-70 V

Operating temperature 200 degrees C

Countervoltage 1.5 V

Heater 115V AC 400W

Temperature range 160-240 degrees C

Constant temperature precision ±5 degrees C

Dimensions 240X169X150mm

Weight 3.5 kg

Franck-Hertz power supply
This power supply unit is designed for operating either the Ne or Hg filled Franck-Hertz tube. It provides all the voltages needed to power the tubes and includes a sensitive built-in DC amplifier for measuring collector current. The accelerating voltage can be provided manually or be means of a saw-toot output voltage from the power supply unit. Additional measuring inputs are also available for the anode current and accelerating voltage.


Filament voltage 4-12 continuously adjustable

Control voltage 9 v @ 10 mA

Accelerating voltage 0 –80 V

Operating modes manual, continually adjustable Saw-tooth (60 Hz)

Countervoltage 1.2 – 10 V continually adjustable

Analog outputs:

Signal output 0 –12 V @ 7 nA/V

Accelerating voltage UA/10 for oscilloscope

Connections via 4 mm safety sockets

Dimensions 160X132X210 mm

Weight 2.4 kg

(The oscilloscope and connecting cables shown are not included as part of the system.)