Franck—Hertz Experiment

Abstract

In 1913 Bohr took Rutherford's nucleus model of the hydrogen atom as the basis for his quantized atom model (► Bohr's atomic model; Rutherford atom). Although it was not the first, it was the first successful atom model. A year later, two Berlin experimenters, James Franck (1882--1964) and Gustav Hertz (1887--1975), unaware of Bohr's model and its implications, performed an experiment which later turned out to be one of its strongest corroborations. For the so-called Franck---Hertz experiment, they were awarded the Nobel Prize for Physics in 1925. In this experiment ► electrons are ejected from a cathode, C, into a tube filled with mercury gas (see Fig. 1). The energy of the electrons can be increased in a controllable manner by accelerating them towards the positively charged grid, G, through the potential difference Va. Electrons fly through the grid towards anode A. Between G and A, a small retarding voltage, Vr, decelerates the electrons. They will only reach the anode A, if their energies V exceed Vr, where they will be recorded by the ammeter A.