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First Production Method
The first Osira lamps were made entirely by hand, starting out essentially with just a plain glass tube. This would be domed over at one end to form a test-tube shaped object, the dome then being pierced with a small hole through which one of the electrodes was inserted, and sealed in by a glassblower with the aid of a gas torch and blowing pipe. Blowing was accomplished through the opposite end of the arc tube which was of course open. Another small hole was then blown beside the first seal, and the auxiliary starting electrode sealed in. Following this yet another hole was made and a short length of exhaust tubing was attached.
Attention was then directed at the opposite open end of the arc tube which had next to be domed over just as was done to the first end. By transferring the blowing pipe to the exhaust tube, a hole could be blown in the heated dome at the second end of the arc tube. The second main electrode was then sealed in, this completing the physical construction of the arc tube. With all processes being carried out by hand a considerable amount of effort went into making the early lamps, reflected in their high sale price.
The Blown-Bulb Arc Tube
BTH pioneered a method which considerably increased the speed with which MA arc tubes could be manufactured through a most ingenious innovation. Instead of building the arc tubes from straight glass tubing, the company pre-formed arc tubes to a near-final shape by blowing the glass into a specially shaped mould on automatic Westlake machines. It is easy to tell an arc tube which was made by the improved BTH process, because feint circular lines appear around the circumference of the arc tube, these impressions being left in the glass as a result of it being rotated inside its mould on the bulb-blowing machine. With early arc tubes made from drawn tubing, if there are any lines present in the glass surface they will be running parallel to the axis of the tube, these having been formed as a result of the tube drawing process.
The only tasks requiring manual labour involved the piercing of three small holes in the dome end of the arc tube bulb, the main electrode, auxiliary electrode, and exhaust tube being attached one at a time after each hole was blown. Next, the final electrode was sealed into the constriction at the opposite end of the arc tube, this being a swift and easy process since the dome-ended shape here was almost completely formed. Final arc tube processing was then carried out on an automatic rotary exhausting machine which pumped, baked, illuminated and gas-filled the arc tube. The human touch was retained, however, for the tipping-off process in which the exhaust tube was melted over by hand-torch to remove the arc tube from the pumping machine - the precise shape of the tip being quite important for good lamp performance. The principal production stages for the blown-bulb arc tubes are illustrated below.
Figure 33 - Production of MA Arc Tubes from Machine-Blown Bulbs
Lamps made with these machine-blown bulbs proved to be quite superior to the earlier models. Aluminosilicate tubing was always hand-drawn prior to this development and despite the fact that a pair of skilled glassblowers could control the diameter of their tubing to within a millimetre of the required size, the tube was often not completely round or might have a slight taper in the wall thickness along its length. This affected the temperature distribution in finished lamps which affected the mercury vapour pressure, and the spread in luminous flux from one lamp to the next was often rather large as a result. The consistency of machine-blown bulbs greatly reduced this variation in lamp performance. As a result BTH was able to increase the average efficacy slightly, knowing that very few if any of the new lamps would be likely to be overloaded, as might have been the case with the uncertainty of hand drawn tubing.
Arc Tubes for Automatic Sealing
It was Siemens Bros. of Preston who completed the task of mechanising the production of MA arc tubes, through a variation of the aforementioned BTH process. Their method was later copied by Osram-GEC and Crompton, and by Westinghouse in the USA. The shape of the blown-bulb arc tube was modified to include a dimple at its crown. At the lamp works the tip of the dimple was removed, and the lower cullet portion similarly cut away, leaving an arc tube with two open ends having reduced diameters.
One end of the arc tube was next lowered over the electrode assembly in a special pinch-sealing machine. Heat was applied to the constricted end of the arc tube and when the glass had reached its softening temperature, a pair of hammers softly pinched it flat around the lead wires thus forming a neat, good quality seal. The same was repeated at the opposite end of the arc tube and by this method, the production of MA lamps was greatly de-skilled. The consistency of electrode positioning improved and so did lamp performance. Because it was found too difficult to include an exhaust tube in the pinch-seal, this was attached to the arc tube side, again with the aid of a mechanical jig to pierce the hole and then fuse an exhaust tube over it.
Figure 34 - Production of MA Arc Tubes by the Pinch-Seal Technique
Manufacture of Special Lamps from Tubing
For lamps of unusual length or special diameters, a different manufacturing technique was employed since it was not economical to fabricate moulds into which semi-formed bulbs could be blown automatically. This production method was standard practice on the 1000W and 2500W lamps as well, which were not required in such large quantities. The arc tubes were made from drawn tubing, which was first heated at its centre while revolving in a glassblowing lathe. The heated central portion would be constricted inwards and the tube parted into two halves at this point, each half being left with a neatly domed end.
A main electrode was then sealed into one half of the tube, and the other electrode, auxiliary, and exhaust tube attached to the other half of the tube. The two halves were then brought back together again and sealed together at their opposite ends to form the completed arc tube, as in Figure 35 below.
Figure 35 - Production of High Watts MA Arc Tubes from Drawn Tubing
Glass-to-Metal Seals
To form a good seal between glass and metal, there are two conditions that must be satisfied. Firstly the softened glass must 'wet' the metal and adhere to it, and secondly the stresses developed by the different rates of expansion and contraction of the components must not exceed the strength of the glass such that the seals crack or leak. Molybdenum and tungsten have similar coefficients of thermal expansion to aluminosilicates, molybdenum being the higher of the two metals. Although tungsten is a better match in terms of the stated expansion coefficients, molybdenum has always been the preference in practical lamps. This is because the glass and metal do not heat up at the same rate when the lamp is switched on - heat is generated at the electrode which is welded to the leading-in wire and consequently this warms up more rapidly than the glass surrounding it. It is found that molybdenum seals are less likely to crack during warming up because of this situation. Glasses used in all but the first MA lamps typically had an expansion coefficient of around 43x10-7 m/°C.
In most MA lamps the seals consist of ground moly wires 1.0mm (0.040") in diameter. The wire is of a specially prepared type to ensure that it is free of splits which might allow air to leak into a finished lamp, and thoroughly cleaned usually by an electropolishing method and by furnacing in hydrogen or under vacuum to remove included gases which might otherwise evolve bubbles in the glass seal during formation, possibly leading to cracking. However glasses do not wet pure molybdenum and stick to it, the seal is actually made to a thin layer of oxide of a precisely controlled thickness, formed by first bringing the metal to a cherry-red heat in a small gas flame. The wire is then beaded with a small glass sleeve of the same composition as the arc tube glass, the oxide layer dissolving into the bead and forming an excellent bond. The wire displays a metallic brown or sometimes deep gold colouration if the seal has been formed correctly. It is necessary to pre-sleeve the wires in this fashion, they cannot simply be sealed directly into the arc tube. The level of heat involved in sealing them in is so high that further oxidation would take place and an over-oxidised seal would be formed which would be quite likely to leak in service.
The shape of the glass bead so formed is all-important as this determines the likelihood of a crack forming during lamp life. Some minor cracking is often inevitable and the shape of the bead and the distribution of internal stress must be controlled so that if a crack does form, it will not penetrate right through the wall of the lamp and cause an air leak (Scott, 1945). In final lamp assembly, the sleeved lead wire would then be sealed directly into a small hole which had been formed in the end of the arc tube. Seals of this type were successfully fabricated with wires up to 2.0mm diameter (0.080") for the manufacture of experimental 2,500 Watt MA lamps.
Arc Tube Exhausting
The exhausting process of MA arc tubes was essentially the same for all of the different arc tube construction methods illustrated above. To remove all foreign impurities it was necessary to heat the arc tube to a temperature much higher than its operating temperature while under high vacuum, typical outgassing temperatures being of the order of 700°C or so. In the early days arc tubes were heated with a simple gas flame from a hand torch whose flame was cautiously played over the whole arc tube. As production volumes increased rotary automatic pumping machines were devised, very much along the lines of the equipment that had been developed for processing fluorescent tubes. Arc tubes went through several stages on this equipment. First they were pumped to rough vacuum, the vacuum quality gradually increasing as the bulb was heated to as high a temperature as possible without it sucking-in under the vacuum. They were then filled with argon and a high voltage high current AC discharge established in the lamps to render the electrodes brightly incandescent, thus activating the emissive pellet and driving out impurities. After the baking, pumping and bombarding process the arc tube was given its final dose of spectroscopically pure argon.
Throughout the entire process the tiny dose of mercury had to be kept out of the ovens and in a T-shaped appendage on the side of the exhaust tube - had it been heated it would have vaporised and been pumped away. When the arc tubes were hand-tipped off the exhausting machine, they were long-tipped above the point of the side T-piece. Once off the pump the lamp could be tilted and the mercury dose allowed to run into the arc tube. The long tip and side T-piece were then removed with the mercury inside the arc tube, the shape of the final tip-off being formed by skilled operators with the aid of gas torches.
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