||Following the introduction of the original 400W and 250W MA lamps, additional ratings were developed. Owing to the fact that the efficacy drops rapidly below 250W, which made a briefly-produced 150W rating rather unpopular, most effort was focussed on higher power lamps - beginning towards the end of the 1940s. Although ratings of 650W, 1000W, 2500W and 3000W were developed, it was only this 1000W lamp that became relatively popular in Britain, and the large 3000W version in America. The MA/H 1000W was introduced in the mid 1950s and its primary application was for industrial floodlighting and photographic illumination.
At such high power loading, the arc tube is much less susceptible to draughts of cold air and as a result the normal glass outer envelope has been dispensed with. The arc tube is fabricated in BTH type C14 aluminosilicate glass. In fact, it has been produced from two 400W arc tubes joined together, as evidenced by the glass-glass seal visible at the centre of the lamp, and a surface texture that is consistent with mould-blown rather than drawn tubing.
The electrodes employ a thorium oxide pellet as the emissive material, enclosed in two perpendicularly wound tungsten coils. At one end of the lamp is a tungsten probe auxiliary electrode. The glass-to-metal seals employ molybdenum rods and at one end of the arc tube there is an exhaustion tip. Each end of the arc tube bears four small dimples in the glass, to facilitate the cement bond to the caps. The caps consist of vented metal cylinders, and via a piece of BTH Syndanyo insulator carry the end terminals. One of these is annular, the centre contact being connected via a high resistance to the auxiliary ignition electrode.