English: Ionization energy of atoms is measured in the gas phase. As the wavelength of the impinging light is swept down the ultraviolet range, light quanta reach energy high enough to dislodge the least bound electrons. These will be attracted by the positive electrode, and the remaining positively charged ions will travel toward the negative electrode. The ionization energy will be the energy of photons that caused a steep rise in the current of electrons and ions established in the tube. The atoms can be ionized by a beam of electrons of a known energy in a similar setup.