SO/H Positive Column AC Lamps

The first U-shaped positive column lamps were put on the market in 1932, a very busy year in which three generations of sodium lamp were introduced and promptly superseded! They were known as type SO/H, SO denoting SOdium vapour and the /H implying that the lamp had to be burned horizontally For the first time in this year, the solid borate glass was abandoned because it was expensive and so difficult to work with, and it was succeeded by an ordinary soda-lime glass tube having a thin layer of borate blown onto its inner surface The tube was bent into a U-shape, and an oxide coated electrode was sealed into each end of the tube using the reverse-pinch kind of seal that Philips had invented for its DC lamps. The lead wires were soldered to the base contacts of a porcelain bayonet cap, and an enamelled iron fork gave mechanical support to the U-tube and located the bend inside a dimple in the inner side of the dewar jacket The fork was electrically connected to one of the electrodes and served as a third auxiliary external electrode to facilitate ignition of the discharge.

Ifthe discharge tube was run in free air, thermal losses would prevent it from getting hot enough to vaporise the sodium with the effect that the lamp would never run up To achieve the optimum temperature of 260ºC without providing an external heater, some form of thermal insulation was required The lamp with the best insulation would be the most efficient since less power is needed to keep it warm.  Heat is lost from the discharge tube by conduction, convection and radiation and the best lamp design minimises each of these three loss mechanisms.

Conduction and convection losses were greatly reduced by a relatively simple approach - the discharge tube was mounted inside a separate, detachable dewar vacuum jacket Heat is conserved as it cannot be conducted or convected across the vacuum space in the jacket The principal is just the same as when we use a Thermos vacuum flask, say, to keep a drink of coffee warm.  However SO/H did little to combat the loss of radiated heat, and conduction and convection losses from the discharge tube to the air space around it prevented efficacy from increasing further An SO/H lamp is shown diagramatically in Figure 15 and some examples have been photographed and can be viewed with individual details from the main page of this website

Figure 15 - Diagram of the SO/H Dewar Style Lamp

The first SO/H lamps were offered by Philips in four wattages, details of which can be found in Table 1 below All lamps were rated for 2,500 hours service. (Dorgelo and Bouma, 1937) It is not stated whether these figures apply to 100-hour values or average through-life performance, but the former would seem to be the more common figure for lamps of this era.

Table 1- Specifications of the First SO/H Style Lamps

Soon after 1937, a new glass type was made which offered a reduced rate of argon absorption and lamps need not be given such a large dose of this gas initially The new gas filling increased lamp efficacy by a small amount, and the lamps were re-rated to lower wattages to keep the lumens roughly the same for each size The data for new re-rated SO/H lamps is quoted in Table 2, these being 100-hour values(Philips UK Catalogue, 1943).

Table 2- Specifications of the Re-Rated SO/H Style Lamps

One of the principal problems with all dewar-jacketed lamps was that as they cooled after switching-off, the air around the discharge tube contracted thus drawing in cold air from outside This often carried dust into the lamp, which gradually built up and absorbed light In addition if moisture was drawn in, this would form light-blocking condensation films and being an electrical conductor, wet lamps were often very difficult to strike up in the evenings In the early days public lighting engineers would walk the streets on humid evenings and manually switch on and off the lantern in question in the hope that it would strike up after a few attempts.

BTH-Mazda was the only firm to attack these issues and their lamps were the favoured brand for many years A pair of sachets of silica gel were attached to the U-bend of the lamp to absorb moisture, and in the 85W rating which was the most difficult size to start up owing to its long tube of slim diameter, the glass was coated with a water-repellent silicone film At one point, the company also included a xenon component in the gasfilling of the 85W lamp because the this rating has the highest electrical loading on the glass, and argon absorption by the glass was causing premature lamp failure It was first discovered by Osram-GEC that xenon is not absorbed by the glass and the BTH 85W lamps gave considerably longer life, although the high atomic weight of xenon did result in the loss of some 12% lumens.

In 1955 a new kind of glass was introduced by Philips which offered considerably enhanced resistance to sodium corrosion A disadvantage however, was that liquid sodium exhibited rather poor adhesion to its surface and if not kept absolutely level, it would flow around the lamp forming large light-blocking mirrors Although it improved lumen maintenance figures due to reduced browning of the glass, this was largely offset as a result of the formation of sodium mirrors To combat that problem, the so-called "Bamboo" lamp was invented by Philips and also manufactured very briefly by Osram-GEC a few years later The glass was rilled in at several points giving it the appearance of a bamboo cane, and the ridges were successful in preventing the liquid sodium from flowing around the lamp (Figure 16).

Figure 16 - Bamboo type construction employed in post-1955 Philips lamps

The new glass had another drawback though, in that it absorbed argon at a much faster rate than the previous composition which would become stained brown rather rapidly To attain a useful lamp life, the rare gas filling was changed at this time to a neon-xenon-helium mixture which has more elastic and inelastic energy losses That reduced discharge efficacy slightly but over time it was more than offset by the greater light transmission of the new glass which did not stain so rapidly Lamp efficacy for the 140W model rose from 76 to 84 lm/W as a result.

Meanwhile BTH Mazda set up its own production of a sodium resistant hard glass which marked an important step forward for that company The borosilicate 2-ply tubing it created was chemically stable while also having good adhesion to liquid sodium and a fairly low argon cleanup rate Thus their lamps, except the 85W as noted above, could operate at up to 87lm/W for the 140W rating A further advantage, and in fact the main reason why that hard glass was developed, was that it had a lower coefficient of thermal expansion than the Philips/Osram soft glasses and the occurrence of cracking in production and service was almost totally eliminated There was a severe price penalty for the use of this glass though, and once an improved soft glass had been developed, production reverted to that style and the greater production scrap rates associated with soft glass were accepted It is probably fair to say though, that the hard glass SO/H lamps made by BTH during the years 1951 to 1958 were the best quality sodium lamps produced in that era.

The argon cleanup problem of the new soft glass was solved in 1958 by treating the glass with excess argon during manufacture, and in that year the efficacy of Philips and Osram sodium lamps from 84 and joined the 87lm/W figure of BTH because they also could re-adopt the former neon/argon gas filling In the same year Philips pioneered the introduction of small dimples in the sides of the discharge tube, which formed cool spots and served as sodium reservoirs to hold the sodium in place Due to the fragility of the exposed dimples, they were only employed in SO/H detatchable lamps for a brief period, and Philips quickly adopted the 'Integral' design invented by Osram, which is covered in the next section.