In this lab, the student will recreate the Franck-Hertz experiment which provided early evidence of the quantization of energy levels in atoms.  The experiment involves watching a low pressure gas in a tube as energetic electrons collide with the gas.  At first, when the electron energy is increased from zero, the current increases because the electrons have a higher probability of overcoming the potential barrier near the electrode.  At a certain accelerating potential the electrons have just the right amount of energy to excite the atoms in the gas to higher energy states.  When this occurs the electrons can not penetrate the potential barrier as easily, and the current will decrease.  Also, photons will be emitted by the gas atoms at wavelengths corresponding to the energy level to which the electrons have excited them.  The student begins the experiment by setting up the apparatus as shown in the lab handout.  The gas tube used in this experiment contains neon and mercury gas.  Starting at zero, the student should slowly increase the accelerating voltage until the first transition is seen, noting the accelerating current and collector current.  The student can then use a spectroscope to view the spectral line at this transition.  Then the student should continue increasing the accelerating voltage, recording all transitions seen.  Once finished, the student will then need to graph the collector current versus the accelerating voltage, and interpret the graph and all the dips and peaks present.  Also added to this graph should be the wavelengths of the spectral lines observed at the corresponding accelerating voltage and collector current.