||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 by creating an arc between electrodes fabricated of the metal to be studied, or by sprinkling a powdered salt into a 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 delivered much more stable results by virtue of the high purity and constant output.
Five lamps make up the group containing Bivalent Metals in the Philips series. These are Mercury (both high and low pressure), Cadmium, Zinc, and a mixed lamp containing all three elements. The low pressure mercury lamp is similar lines to the noble gas family. The rest of this group is characterised by a higher vapour pressure of the metal dose, achieved with compact arc tube of high wattage.
The electrodes and arc tubes resemble those found in low wattage mercury discharge lamps. It is standard practice to form narrow diameter quartz diaphragms just in front of each electrode, to constrain the arc along the central axis of the tube without wandering and flickering. Heat-reflective coatings of platinum paint are applied behind the electrodes to further raise the vapour pressure of the metals. The arc tube of the zinc lamp is further stretched into a capillary at its centre to raise the intensity of the discharge. Seals were originally formed to tungsten rods, as in the first mercury lamps but in later models the standard pinch-seal technique is employed. Outer bulbs are fabricated in hard glass to withstand the heat, or quartz if UV transmission is required.