The first truly successful mercury lamp to operate at a high pressure was invented and developed by The General Electric Company of England during the years 1923-1932. It was marketed later that year, although remarkably the fact that it employed mercury vapour was kept secret until 1933 when lamps became available in quantity and competitors began to introduce their own equivalents. Until that time it was known simply as the "Osira" Luminous Tube lamp. In later terminology it was classified as the MA lamp, 'M' standing for Mercury and 'A' signifying a medium-pressure discharge operating in a glass tube with a power loading of more than ten watts per centimetre of arc length. Incidentally the name Osira was introduced because until the second world war, the GEC was only licensed to use its Osram brand name on metal filament incandescent lamps. It had been granted that right by the German firm Osram many years earlier because the GEC was responsible for manufacturing and marketing metal filament incandescent lamps according to Osram's design throughout the British Empire - but it was only permitted to use the name on those products. This imposition fell at the time of the second world war and thereafter, all GEC lamps bore the Osram name.
The GEC knew that mercury lighting could be efficient on account of the performance of the lamps made by Cooper-Hewitt and Küch & Retschinsky, but neither of those products was of practical use for general lighting applications.
In 1923 a research effort began at the Hirst Research Laboratories in Wembley to improve upon these crude early designs and develop a new lamp which would overcome most of their shortcomings, creating a market for itself as a real alternative to the incandescent lamp in everyday applications. It was quickly identified that the following key points would have to be satisfied in order to realise such a lamp:
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A mercury vapour pressure of at least one atmosphere
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Complete vaporisation of the whole mercury dose for improved lamp stability
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A glass tube capable of continuous operation at a minimum of 600°C
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Highly emissive electrodes to carry a high lamp current for at least 1000 hours
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A means of insulation to protect the discharge tube from draughts of cold air
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Compact dimensions to facilitate its use in standard optical systems
The GEC scientists theorised that such a lamp would attain a luminous efficacy 3 times greater than that of the tungsten filament lamps, and that its colour would be improved to the point that it was suitable, at least, for street and industrial lighting. They promptly set about designing a lamp along these principles which would be a satisfactory replacement for the thousand-watt tungsten lamp, the workhorse of industrial and area lighting at the time. Many other discharge lamps were conceived during this period of intense research, the basis of the fluorescent tube and the hot-cathode neon lamp to name a few, but the real fruits of their work came in 1932 in the form of the "Osira" mercury lamp pictured below.
Figure 19 - The First Mercury Lamp for General Lighting - Invented by British GEC, 1932
(Courtesy of National Museum of Scotland, Edinburgh, UK)
At the heart of the lamp is an arc tube fabricated from a new Aluminosilicate glass developed by the GEC specifically for this lamp. The electrode requirements were met through a modification of the old Wehnelt cathode, and consist of a pelleted mixture of barium, calcium and strontium silicates held in a heavy coil of tungsten wire. The spacing between the electrodes is 182mm, the inside diameter of the arc tube 34mm, and it contains 5 torr of argon plus a few milligrams of mercury. In operation the entire mercury dose is vaporised and the arc tube attains a temperature of around 550°C, the coldest spot still being in excess of 360°C and this delivers a mercury vapour pressure of precisely one atmosphere (nearly four times greater than had ever been achieved in a practical lamp). Because all of the mercury is vaporised in operation, changes in ambient temperature or supply voltage have very little effect on lamp performance because they cannot cause more or less mercury to be vaporised into the discharge. The arc tube is housed in a sealed, nitrogen-filled tube to protect it from cold draughts of air which could influence the mercury pressure (note the pip top), the resulting lamp being equipped with a GES (E40s) screw cap and measuring 50mm in diameter by 300mm long. |
