THE FRANCK-HERTZ EXPERIMENT

In this experiment we measure the ioinzation energy of the Hg atom by observing
the absorption dips in its electron collision spectrum. The experiment showed for
the first time that quantum levels existed in atoms.

Franck-Hertz Apparatus

(1) Turn Franck-Hertz Oven to T=150 degC and let it warm up.

(2) Turn on the oscilloscope, DVM (thermocouple.) and amplifier/voltage unit.

(3) The oven uni is hot. CAUTION!

(4) The electrical box has an amplifier and heater unit. I have marked the nominal settings. Use
them to begin with.

(5) Observe the Hg vapor resonance dips on the oscilloscope. The scope is running in X-Y

mode with X=accelerating voltage /10 and Y=Anode (collector) current. The accelerating voltage is

divided by ten. The 60V x-sweep should appear as 6V on the oscilloscope!.

(6) Position the scope time cursors on the dips and measure spacing. Wiggle the cursors a bit to determine an error on each measurement.

(7) The oven cycles. Record the temperature through two tcycles each 30 seconds and determine if
this is a source of systematic error on measuring the spacing.

Analysis

(1) Average your measurements and determine the first Iionization Potential energy of Hg.

(2) Repeat the measurement at Toven = 180degc.

(3) Do your measurements agree with spectroscopic data on Hg? See the article provided in your

lab manual. Add a comment to your Conclusion!

Questions: (Please add to your lab report.)

(1) Why are ther multiple dips in the accelerating vs. collector current spactrum?

(2) What is a thermocouple and how does it work?