THE FRANCK-HERTZ EXPERIMENT

LIBRARY RESEARCH. Research the Franck-Hertz experiment. Identify what quantities you would want to measure and what variables there might be to control. Be prepared to explain what differences there would be between a neon Franck-Hertz tube and a mercury one and why.
(a) Franck-Hertz power supply, (b) Neon Franck-Hertz assembly, (c) Mercury Franck-Hertz assembly.
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NEON FRANCK-HERTZ TUBE:
WIRING:

• Make sure the control unit is OFF and all knobs are turned all the way counter-clockwise.
• Connect the control unit to the Franck-Hertz tube by matching the plugs on the lower panel of the control unit with the corresponding plugs on the neon F-H unit Put an AC ammeter (with a needle dial) in series with either the H or the K wire.
• Use a coaxial cable to connect the BNC plugs on the tube and the control unit.
• Have your instructor show you how to connect a storage scope to the control box. This will allow you to capture a graph of the two quantities you are measuring.
• Turn on the power to the control unit. Turn on the scope and make sure it is in XY display mode. (x: 1.00V/div, y: 2.00V/div)

EXPERIMENT:

• Gradually increase the Heating ("Heizung") to about 8V. Do not change this setting for this tube.
• Make sure the toggle switch beneath the accelerating voltage knob is turned to "Ramp" rather than "Manual". Increase this knob to about 30V.
• Increase the amplification (topmost knob) to about half of its maximum value.
• Slowly vary the Reverse bias until you see sharp distinct peaks and valleys on the scope.
• When you start to get peaks and valleys on the oscilloscope, increase the accelerating voltage to about 60V or 80V. Vary the bias to ensure that you have sharp peaks and valleys. Choose scales that give you a nice graph. Print out a copy of the graph for your notebook.
• To measure the positions of the peaks and valleys, scroll the graph to the left (rotate the X position knob) and record the value by which the x-position has been moved. The actual accelerating voltage is 10 times this value. Discuss with your instructor how to analyze these data.

MERCURY FRANCK-HERTZ TUBE:
WIRING: Follow the steps below in order. It is much easier to wire the circuit correctly if you think of it in modular units rather than trying to do it all at once. Call your instructor over to look at each part of the circuit before you plug it in. BEFORE YOU BEGIN, make sure everything is turned off (except the oven) and that all knobs are turned all the way counterclockwise.

• HEATER. Plug the power cord of the Franck-Hertz apparatus into a variac (a variable-voltage power supply). Turn on the variac, and turn it all the way up. Turn on the thermometer. The oven should eventually rise to about 160^oC-200^oC. You will adjust the variac to vary the temperature.
• OSCILLOSCOPE. Keep this connected to the Control Unit. Turn it on as before. You should have a single bright dot on the screen.
• CATHODE HEATER. Move the "H" and "K" wires from the Neon Franck-Hertz tube to the lowermost two plugs on the Mercury Franck-Hertz tube assembly.
  • Turn on the power and give the heater 6V (dial reading). Keep an eye on the ammeter to ensure that the current doesn't exceed 0.3A.If the tube is okay, you should see a weak glow from the top of the cathode. Verify. Describe the glow in your write-up.
• ACCELERATING VOLTAGE. Connect the tube's grid to plug "A" of the power unit. This allows you to accelerate the electrons. Make sure the voltmeter knobs are turned all the way counterclockwise before turning it on. Limit this to about 40V maximum.
  • Turn on the accelerating voltage, and bring it up to about 20V. Turn off the lights and look for a glow above the orange glow of the cathode. Describe it in your notes.
• ANODE. The anode is the electron 'collector'. We can count how many electrons arrive here by measuring a current.
  • Connect the "BNC" coax connector of the control unit ("M" plug) to the BNC plug on the Franck-Hertz tube assembly.
  • The Reverse Bias ("Gegenspannung") reduces the total output current, to reduce the wear and tear on the tube. Start with this knob turned all the way counterclockwise. If the current on the scope is "clipping", adjust this knob to reduce the current. You can adjust the Amplitude knob at the top of the control unit to better see the current on the oscilloscope.

EXPERIMENT.

• Sketch the peaks and valleys that appear on the scope in your notes .
• Choose appropriate scales on the oscilloscope to get a good graph, then print a hardcopy of this graph, recording (a) the temperature and (b) the current scale from the oscilloscope.
• Repeat for at least three different temperatures between about 160^oC and 200^oC, adjusting the variac to tune the temperature.
• Locate the peaks and valleys in the same way you did for the neon Franck-Hertz tube.
• Calculate the spacing between peaks and valleys for each temperature, with uncertainties, and compare them.

ANALYSIS:
Use the voltage values to test whether the voltage peaks are evenly spaced in voltage, and what the spacing is. Compare to the spacing for neon.

Also, describe in your lab writeup what effect temperature has on the amount of current flowing in the tube at various voltages for the mercury tube. Explain what causes this effect. (Hint: think about what role the oven plays.)

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