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 V _a. Electrons fly through the grid towards anode A. Between G and A, a small retarding voltage, V _r, decelerates the electrons. They will only reach the anode A, if their energies V exceed V _r, where they will be recorded by the ammeter A.