||Spectral lamps find a home in laboratory applications where they are commonly employed as stable, high quality sources of discrete spectral lines. Atomic spectra were originally produced either by creating an arc between electrodes fabricated of the metal to be studied, or by sprinkling a powdered salt into an ordinary gas flame. Both methods produce somewhat unstable results and require constant attention. In the 1940s a range of electric discharge lamps was developed to supersede these crude methods, and could guarantee superior results by virtue of the high purity of the metals contained within the discharge tube.
Lamps containing the noble gases do not require a special quartz discharge tube owing to the non-corrosive nature of their gas fillings, and the fact that their electrical loading is not especially high. A simple borosilicate discharge tube is satisfactory, and seals are formed to the usual iron-nickel-cobalt alloy as found in other alkali metal lamps.
The electrode assembly depends on the electrical loading which is developed in each gas type. Most lamps employ a simple backwound coil of thoriated tungsten containing a triple carbonate emitter impregnated within the coils. The Helium lamp is different since this dissipates a rather high power in its discharge, and here a high-current electrode is used, of a similar construction as is found in the sodium lamp. A thin coil of tungsten wire is wound around a pellet of thoria which imparts excellent electron-emissive properties to this electrode, minimising its operating temperature and allowing it to handle a higher current density. The helium lamp is also provided with an internal glowbottle starter to facilitate ignition of the discharge.