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Lamp Glass Requirements
Developments in glass technology, a field in which the GEC had tremendous expertise, are what ultimately led to the feasibility of the MA lamp. In the Cooper-Hewitt lamps borosilicate had been employed, but it was not possible to run this at a particularly high temperature and consequently a high mercury pressure could not be built up. Quartz was known from the Spanner and Bastian lamps which evolved from the Küch & Retschinsky developments, and although this was more than suitable in terms of its refractory properties, its thermal expansion coefficient was so low that no satisfactory method of sealing metal conductors through it was known. The techniques which were employed were all extremely tedious and frequently cracked or leaked, thus the quartz lamps could only be used in applications where they did not have to be relied on as a source of light, confining them mainly to medical and scientific applications.
GEC Glass Development
The GEC Glassworks at Lemington and the glass technologists at Wembley were therefore charged with developing a glass which was more refractory than borosilicate, but which still had a high coefficient of thermal expansion so that metal wires could be sealed through it. It was also required to have a high electrical resistivity at the elevated temperatures it was to operate at, and it had to resist any chemical attack from superheated mercury vapour and oxide coatings originating from the electrodes. This occupied the bulk of the period spent in developing the MA lamp, but was outstandingly successful. A new family of glasses called the aluminosilicates were developed expressly for this application, resulting in the GEC's popular H26X glass type.
Essentially the principal component of borosilicate glasses, boric oxide, had been substituted with aluminium oxide. This had the effect of increasing the softening temperature by a crucial 200°C, and the coefficient of thermal expansion actually rose slightly above that of the borosilicates. It was found to be perfectly suitable for sealing directly to lightly oxidised molybdenum wire. The higher expansion awkwardly made the glass more sensitive to thermal shock, and in consideration of this protracted annealing schedules had to be introduced every time the glass was heated up to softening temperatures. In this respect aluminosilicate is perhaps the most hated material of glass technologists, it seemed you only had to frown at a batch of finished lamps and they'd promptly crack! Nevertheless with care, it could be worked and techniques were found to reliably seal molybdenum conductors through it.
BTH Glass Development
Shortly after Osram showed the Osira lamp, British Thomson-Houston became active in making aluminosilicate glasses and was soon manufacturing its own C14 grade at the Chesterfield glassworks. The expansion coefficient was 37x10-7 m/°C and the softening temperature was 730°C, derived from a glass comprising 58.5% Silica, 22.5% Alumina, 3.0% Boric Oxide, 15.2% Oxides of Mg, Ba and Ca, and 0.8% Oxides of Na and K.
In the following years BTH devoted great efforts to further increase the softening temperature of its aluminosilicate glasses, the efforts of J.E. Stanworth and A.E. Dale at the BTH Research Laboratory in Rugby being especially significant. Many of the principal technical papers covering this important period of development were authored by these scientists. In the late 1940's the new Chesterfield C37 glass was introduced, in which the softening temperature was raised to 760°C and this permitted a further increase in lamp efficacy. In 1950 the C46 glass made its debut, again offering higher performance with a softening temperature of 775°C. This last development was important because it meant that for the first time, an efficient lamp which was suitable for universal burning in any orientation could be made. Previously when lamps were burned horizontally, the upward bow of the arc due to convection currents required that an external magnetic deflector be used to prevent the arc from overheating and destroying the arc tube. Until the advent of C46 glass, special lamps were designed for horizontal operation with a larger diameter arc tube and lower mercury dose which meant that their luminous efficacy fell slightly. The new harder C46 glass was just able to tolerate the higher wall temperatures present in horizontally-burned lamps of the standard pattern though, and one universal burning lamp could be marketed which offered full efficiency in either horizontal or vertical orientations.
American Glasses
In the USA the production of aluminosilicates was adopted first by General Electric in 1934, this company having a full patent licensing agreement with BTH so that both companies could immediately make use of each others developments. Corning Glassworks of New York soon took up the manufacture of similar aluminosilicate glasses as well, these being supplied to Westinghouse and Sylvania to enable those firms to also enter the mercury lamp business.
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