||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 Mercury and Cadmium lamps found applications principally in the calibration of spectroscopic equipment, each lamp outputting a small number if high intensity discrete spectral lines at well defined wavelengths, characteristic of their metal fillings. The lamp containing both mercury and cadmium together is particularly useful as it provides a number of well spaced lines covering very nearly the whole of the visible region.
The lamps were popular light sources in schools and Universities of Great Britain and its colonies where GEC was active, with the vast majority being used simply as a demonstration of the spectral properties of different light sources, and how light could be split into its component colours with a prism or diffraction grating.
The construction is based on that of a miniature fluorescent lamp, but with heavier electrodes to withstand the higher current operation. A cathode shield is employed in later designs to minimise blackening and electrode flicker.