electrons keep losing energy, less light is emitted, resulting in another dark space. Anode layer The anode layer begins with the positive column, and ends at the anode. The anode layer has a negative space charge and a moderate electric field. Positive column With fewer ions, the electric field increases, resulting in electrons with energy of about 2 eV, which is enough to excite atoms and produce light. With longer glow discharge tubes, the longer space is occupied by a longer positive column, while the cathode layer remains the same. For example, with a neon sign, the positive column occupies almost the entire length of the tube. Anode glow An electric field increase results in the anode glow. Anode dark space Fewer electrons results in another dark space. Striations Bands of alternating light and dark in the positive column are called striations. Striations occur because only discrete amounts of energy can be absorbed or released by atoms, when electrons move from one quantum level to another. The effect was explained by Franck and Hertz in 1914. Sputtering In addition to causing secondary emission, positive ions can strike the cathode with sufficient force to eject particles of the material from which the cathode is made. This process is called sputtering and it gradually ablates the cathode. Sputtering is useful when using spectroscopy to analyze the composition of the cathode, as is done in Glow-discharge optical emission spectroscopy. However, sputtering is not desirable when glow discharge is used for lighting, because it shortens the life of the lamp. For example, neon signs have hollow cathodes designed to minimize sputtering, and contain charcoal to continuously remove undesired ions and atoms. Carrier gas In the context of sputtering, the gas in the tube is called "carrier gas," because it carries the particles from the cathode. Color difference Because of sputtering occurring at the cathode, the colors emitted from regions near the cathode are quite different from the anode. Particles sputtered from the cathode are excited and emit radiation from the metals and oxides that make up the cathode. The radiation from these particles combines with radiation from excited carrier gas, giving the cathode region a white or blue color, while in the rest of the tube, radiation is only from the carrier gas and tends to be more monochromatic. Electrons near the cathode are less energetic than the rest of the tube. Surrounding the