||ME mercury lamps have air-cooled quartz arc tubes loaded above 100 watts per centimetre, and this 250W lamp operates at a mercury vapour pressure of around 30 atmospheres. The /D suffix indicates that the lamp is for burning in the vertical base down position only, as the shape of the arc tube is often specially profiled for this orientation.
The arc burns between electrodes of solid tungsten to provide a compact source of extremely high brightness. This, the simplest design of ME lamp, has the arc tube housed within a simple hard glass outer bulb and the cap is the standard 5-Amp plug, once the principal mains electricity socket in use in the UK. This very early lamp contains only an argon filling and has no starting electrode. Ignition was accomplished with the aid of a Tesla coil. The arc tube is hand-made throughout. Small cups of pressed nickel behind the electrodes prevent heat losses here and assist in raising the mercury vapour pressure.
ME/D lamps were once very popular high brightness light sources for monochrome slide and film projectors, film printers, and a great many still live on today in technical instruments such as projection microscopes and profile projectors. In addition to this glass design, a lamp having a pre-focussed base was also sold, and a number of types in metal boxes having quartz or glass windows were also made. The metal box types were sold for bare operation, the housing being capable of withstanding the blast should the arc tube explode at end of life. The glass types meanwhile are suitable only for enclosed operation. Today, for new applications they have been totally superseded by the double ended bare arc tube designs.