While it isn't evident in the original measurements of the figure, this series of dips in current at approximately 4.9 volt increments continues to potentials of at least 70 volts.At 9.8 volts a similar sharp drop is observed.The current then increases steadily once again as the voltage is increased further, until 9.8 volts is reached (exactly 4.9+4.9 volts).At 4.9 volts the current drops sharply, almost back to zero.This behavior is typical of true vacuum tubes that don't contain mercury vapor larger voltages lead to larger " space-charge limited current". At low potential differences-up to 4.9 volts-the current through the tube increased steadily with increasing potential difference. The graphs published by Franck and Hertz (see figure) show the dependence of the electric current flowing out of the anode upon the electric potential between the grid and the cathode. The Franck–Hertz tube primarily emits light with a wavelength near 254 nanometers the discharge emits light at many wavelengths. Wavelengths of light emitted by a mercury vapor discharge and by a Franck–Hertz tube in operation at 10 V. On December 10, 1926, Franck and Hertz were awarded the 1925 Nobel Prize in Physics "for their discovery of the laws governing the impact of an electron upon an atom". After a presentation of these results by Franck a few years later, Albert Einstein is said to have remarked, "It's so lovely it makes you cry." The quantization of the atoms matched his formula incorporated into the Bohr model. Therefore, Bohr had followed the instructions given in 1911 and copied the formula proposed by Lorentz and others into his 1913 atomic model. At Solvay, Hendrik Lorentz suggested after Einstein’s talk on quantum structure that the energy of a rotator be set equal to nhv. The relationship of energy and wavelength had also been predicted by Bohr because he had followed the structure laid out by Hendrik Lorentz at the 1911 Solvay Congress. They showed that the wavelength of this ultraviolet light corresponded exactly to the 4.9 eV of energy that the flying electron had lost. In a second paper presented in May 1914, Franck and Hertz reported on the light emission by the mercury atoms that had absorbed energy from collisions. This feature was "revolutionary" because it was inconsistent with the expectation that an electron could be bound to an atom's nucleus by any amount of energy. There were no intermediate levels or possibilities in Bohr's quantum model. This means that the electron is more loosely bound to the mercury atom. After the collision, the electron inside occupies a higher energy level with 4.9 electron volts (eV) more energy. Before the collision, an electron inside the mercury atom occupies its lowest available energy level. Its key feature was that an electron inside an atom occupies one of the atom's "quantum energy levels". The Bohr model was a precursor of quantum mechanics and of the electron shell model of atoms. These experimental results proved to be consistent with the Bohr model for atoms that had been proposed the previous year by Niels Bohr. Slower electrons merely bounce off mercury atoms without losing any significant speed or kinetic energy. A faster electron does not decelerate completely after a collision, but loses precisely the same amount of its kinetic energy. This energy loss corresponds to decelerating the electron from a speed of about 1.3 million meters per second to zero. They discovered that, when an electron collided with a mercury atom, it could lose only a specific quantity (4.9 electron volts) of its kinetic energy before flying away. Franck and Hertz had designed a vacuum tube for studying energetic electrons that flew through a thin vapor of mercury atoms. It was presented on April 24, 1914, to the German Physical Society in a paper by James Franck and Gustav Hertz. The Franck–Hertz experiment was the first electrical measurement to clearly show the quantum nature of atoms, and thus "transformed our understanding of the world".
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