||The GEC family of Spectral lamps consisted of eight basic types. These contained sodium, mercury, cadmium, and a mercury-cadmium mixture, each type being available for AC or DC supplies. The lamps have 4-pin caps such that the cathodes may be electrically heated before starting, to enable the lamps to start easily on the 230V mains electricity supply. A rheostat in series with the mains was usually employed for current control, having an output switched thus: For 30 seconds an excitation current was passed through the cathodes, then the arc was struck and after a further 30 seconds, cathode excitation was turned off.
The sodium lamp is a very convenient source of near-monochromatic light, which has a multitude of scientific and industrial applications. The actual radiation is almost totally radiated in the Sodium Resonance D-lines at wavelengths of 589.0 and 589.6nm. Many special optical phenomena occur under such conditions, for instance optical interference patterns can easily be observed. This lamp was a very popular light source in schools and Universities of Great Britain and its colonies where GEC was active, with the vast majority being used simply as a light source to demonstrate the classic Young's Slits experiment. The lamp also had applications in producing interference fringes which could be viewed to determine the flatness of a surface, the flow of liquids etc.
These later model lamps are based around SOI lamps of the period, but with an exhaust tube sealed into the pinch at one end. The heat-reflecting glass sleeve has calcium carbonate masks at the ends to which simply function to block flickering light at the cathodes.