The  Franck-Hertz Experiment used to excite atoms by low energy electron bombardment. The energy transferred from electrons to the atoms always had discrete values. Thus the existence of atomic energy levels put forward by scientist Bohr. It is a very important experiment and can be performed in any college or University level laboratory. The Experiment is consists of the following: Argon filled tetrode Filament Power Supply: 3.6-3.4V continuously variable Power Supply for VG1K: 1.3-5V continuously variable Power Supply for VG2A: 1.3 - 12V continuously variable Power Supply for VG2K: 0 - 95V continuously variable Saw tooth waveform for CRO display Scanning Voltage : 0-80V Scanning Frequency : 115±20 Hz Multirange Digital Ammeter Display : 3½ digit, LED Range : 10-7, 10-8 & 10-9 A The instrument can, not only lead to a plot of the amplitude spectrum curve by means of point by point measurement, but also directly display the amplitude spectrum curve on the oscilloscope screen. This instrument is used as a classroom experiment  for demonstrating  to a group of students.