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Demonstrate that mercury atoms are excited only in discrete energy quanta by accelerating electrons through a tube containing mercury vapor, and observing the current of electrons striking the opposite side of the tube as a function of the accelerating voltage.

When the cathode is heated, electrons are emitted and accelerated toward the positively charged Anode. Once they pass through the Anode grid, there is a slight retarding potential (reverse bias) and the electrons which overcome this and reach the collector are detected as a small current (on the order of 10^ -11 Amps). As the accelerating voltage is increased, more and more electrons have enough energy to overcome the retarding potential and reach the collector, so the output current increases. Eventually, the electrons have enough energy that, when they collide with mercury atoms, they transfer their energy, so that the mercury atoms are excited and the electrons drop back to a low velocity. This reduces the output current. As the accelerating voltage is further increased, the output current will go up again as more electrons are able to reach the collector after a collision. Eventually the electrons will be accelerated so rapidly that they will reach the excitation energy of the mercury atoms again, and there will be a drop in output current yet again. At high accelerating voltages this process may happen several times.

With the setting above, the accelerating voltage will be driven from zero to fifty volts. Fifty times per second. The collector current can be plotted against the accelerating voltage by connecting these to the vertical and horizontal inputs of an oscilloscope. When this is viewed on the scope, the spacing of successive minima is seen to be a constant 4.9V, which demonstrates that the mercury atoms are excited only in discrete quanta of energy.

If the oscilloscope's horizontal control is set to XY mode, and the X Channel is set to 0.5V per division, then the x-position can be varied until the minima line up with each vertical division. This 0.5 spacing on the scope represents 5V in the accelerating voltage.

Setup Instructions

1. Turn the oven on (it takes about 10 -15 minutes to warm up). The temperature should be around 170 degrees C to 180 degrees C, but the thermostat dial is only roughly accurate so you must use the thermometer inserted in the top to check that the temperature is correct.

2. From the Operating Unit, connect the heater (H) and the Cathode (K) to the front plate of the oven. The heater current affects the amount of current emitted from the cathode. Start by setting it at 10V; 6.3V is the minimum.

3. Connect the Anode (A) to the front plate of the oven, set the switch to Ramp 50Hz, and set the Acceleration Voltage to 50V.

4. Connect the Input (M) to the oven and set the Reverse Bias to 2V. Turn the (output) Amplitude a quarter turn clockwise.

5. Connect the FH Signal Out to the oscilloscope on Channel Y. This is the amplified signal from the collector.

6. Connect the UB/ 10 Out to Channel X on the oscilloscope. This is the accelerating voltage divided by 10.

7. Turn on the Operating Unit. The heater takes about 90 seconds to heat the cathode.

Parts List

Franck-Hertz Oven and Tube in J. Modern Physics, 2A

Franck-Hertz Control Box in J. Modern Physics, 2A

Oscilloscope in L. Misc

Banana Plug Leads in L. Misc

Relevant Simulation

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Other Information

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Real-Life Examples

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More Advanced Reading

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