A simple experiment to study thermionic emission, the Richardson-Dushman equation and the energy distribution function of thermionic electrons emitted from a hot cathode using a triode vacuum tube is described. It is pointed out that such a distribution function is directly proportional to the first derivative of the Edison anode current with respect to the control grid voltage of the triode when the anode voltage is equal to zero. The energy distribution functions obtained are better described by the Fermi-Dirac law than by the Maxwell-Boltzman distribution law. (Contains 1 table and 6 figures.)