Franck-Hertz Experiment

The Franck-Hertz experiment is a fundamental quantum physics experiment (Nobel Prize in physics, 1925) which confirmed the quantization of atomic energy levels. It can be performed with the Leybold equipment for both Mercury and Neon.

  1. Equipment needed.
  2. Franck-Hertz console
  3. Neon equipment
  4. Diagram of Neon setup
  5. Glow from neon deexcitation
  6. Mercury equipment
  7. Diagram of Mercury setup
  8. Mercury Experiment

Caution! All connections to the tubes, etc. should be made before the console is powered on.

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Equipment Needed

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Franck-Hertz Console


From Leybold catalog, purchased from Klinger-Educational Products,112-19 14th Road,College Point, New York 11356, Ph718-461-1822
The Franck-Hertz console provides all the necessary voltage supplies to the Franck-Hertz tubes, the controls for the experiment, and the outputs for measurement.
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Neon Equipment


From Leybold catalog, purchased from Klinger-Educational Products,112-19 14th Road,College Point, New York 11356, Ph718-461-1822
The neon tube does not require heating. It is placed in the Neon tube holder and connected with the DIN cable which provides all the voltages.
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Neon Diagram


From Leybold catalog, purchased from Klinger-Educational Products,112-19 14th Road,College Point, New York 11356, Ph718-461-1822
The neon tube does not require heating. It is placed in the Neon tube holder and connected with the DIN cable which provides all the voltages.
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Glow From Neon Dexcitation


From Leybold catalog, purchased from Klinger-Educational Products,112-19 14th Road,College Point, New York 11356, Ph718-461-1822
When the accelerated electrons excite the electrons in neon to upper states, they de-excite in such a way as to produce a visible glow in the gas region in which the excitation is taking place. There are about ten excited levels in the range 18.3 to 19.5 eV. They de-excite by dropping to lower states at 16.57 and 16.79 eV. This energy difference gives light in the visible range. Since the accelerated electrons undergo inelastic collisions with the neon and are then accelerated again, they can undergo a series of such collisions if the accelerating voltage is high enough. Since the accelerating voltage from the Franck-Hertz console can reach about 80 volts, you can get up to four collisions. This can be seen under proper conditions as four bands of light from the de-excitation in the collison regions.
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Mercury Equipment


From Leybold catalog, purchased from Klinger-Educational Products,112-19 14th Road,College Point, New York 11356, Ph718-461-1822
The mercury tube must be heated to provide enough vapor pressure for the mercury vapor. The console provides the power for the oven, and the temperature sensor is interlocked so that the temperature is controlled.
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Mercury Diagram


From Leybold catalog, purchased from Klinger-Educational Products,112-19 14th Road,College Point, New York 11356, Ph718-461-1822
The mercury tube must be heated to provide enough vapor pressure for the mercury vapor. The console provides the power for the oven, and the temperature sensor is interlocked so that the temperature is controlled.
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Mercury Franck-Hertz Experiment


The diagram shows the Franck-Hertz console with the mercury tube in its heater. The console provides the power for the oven, and the temperature sensor is interlocked so that the temperature is controlled. The temperature sensor can be seen at the back of the oven.



After heating the mercury tube for about 15 minutes, the display of the Franck-Hertz phenomenon could be seen on the scope display as shown. This fairly clean display was obtained after noting that the position of the power leads was important - when they were overlapping each other and the ground lead, you got a doubled display from some sort of pickup.
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