The effect of an electric field on spectrum lines. The electric field may be externally applied, but in many cases it is an internal field caused by the presence of neighboring ions or atoms in a gas, liquid, or solid. Discovered in 1913 by J. Stark, the effect is most easily studied in the spectra of hydrogen and helium, by observing the light from the cathode dark space of an electric discharge. Because of the large potential drop across the region, the lines are split into several components. For observation perpendicular to the field, the light of these components is linearly polarized. The splitting can be easily resolved with a spectrograph of moderate dispersion, amounting in the case of the H_γ line at 434 nanometers to a total spread of 3.2 nm for a field of 10,400 kV/m.