Geissler Tubes

A and B are the electrodes (capillary). The tube contains a rarefied gas under a small pressure of a few tenths of a mm. If a current is passed through such a tube from a high-voltage source, then the current density in the wide parts of the apparatus is small, while in the capillary tube it becomes significant, and the gas shows a sharp glow in these intermediate parts. Such tubes can be filled with different gases, and for each gas the glow in Geissler tubes gives a spectrum characteristic of that gas. Such tubes are used in spectroscopy to calibrate spectral instruments by wavelengths. The spectrum of hydrogen is especially convenient for calibration, giving a series of lines lying in the visible part of the spectrum, starting with red and ending with violet; then helium with a series of lines distributed throughout the spectrum; furthermore, nitrogen, oxygen, and a whole series of other gases give characteristic spectra.