Lamps were an important part of signalling, but their properties are now largely forgotten
A long, serialized article on Railway Signalling in the Railway Engineer (London) for 1881 contains an excellent section on signal lamps. Although the coming of the electric incandescent light is welcomed and regarded as the future, most of the material was necessarily on oil lamps. Most writers on lamps remark on the slow progress in their design, but, in fact, lamps, like the smoothbore cannon, were simple, elementary devices not easily improved upon for practical purposes. The Welsbach rare-earth incandescent mantle that made fuel lamps the equal of electrical ones in brilliance was more than a decade in the future; the efficient Argand burner was well-known, but too expensive for common use. The important change that was taking place in 1881 was the replacement of vegetable and animal oils by mineral oils, which made a significant improvement in illumination while rendering it more economical.
The three components of a lamp are (1) the burner, including wick, reservoir, and means for air supply; (2) the optical system for directing and coloring the light; and (3) the lantern and other protective and supporting parts. The standard of brilliance at that time was the standard sperm candle burning 2 grains per minute. Candles, like the short, fat Admiralty candles used at sea, were already obsolete for railway uses. Candles are not bright enough, and do not burn long enough, to be satisfactory. An illuminating gas burner consuming 5 cubic feet per hour provided about 16 cp (candlepower). A flat-wick burner of 8 or 9 cp might burn for 60 to 100 hours on a gallon of oil. These figures give some idea of the fuel consumption of ordinary lights. Wicks are flat, round or tubular. A flat wick gives better access to air, so it can burn oil at a greater rate and make a more intense light. The tubular Argand wick admits air on both sides of the wick.
The brightness of the flame was the result of incandescent particles of carbon, created when the heat of the flame decomposed the oil, heated by their own combustion with the oxygen of the air. Illuminating gas at the time contained enough heavier constituents to give a luminous flame. Our present natural gas lacks these, and is a pure fuel, useless for illumination (without a mantle) or suicide. The main lamp oils used in Britain at the time were vegetable oils, predominantly rape and colza, though other patent oils were on the market. Railways in the United States used lard oil. Whale oil was very expensive by then, demand for it having eliminated the whales. These oils have a high flash point, 400° F or so, that makes them safe to burn. The flash point is the oil temperature at which sufficient vapour comes off to be inflammable.
Hydrocarbon, petroleum or paraffin oils, as they were variously known, had been introduced to Britain by Young, who distilled the illuminants from oil shale mined near Edinburgh, or oil from wells near Nottingham. This superior illuminant commanded a good price, so was seldom found on railways. American kerosene was much cheaper, in fact becoming cheaper than rape or colza, so it found a ready market. Unfortunately, it was also more explosive, especially when its flash point dropped below 100° F. The problem was poor control of the refining process that allowed too much of the volatile ingredients to remain. Vegetable oils had only to be pressed, and animal oils rendered; they had no volatile components. At first, mineral oils were used only in stationary lamps. The agitation of movement was thought to encourage explosion, so vegetable oils continued to be used in head and tail lamps, as well as for carriage lighting, for some time. The prejudice was still strong in 1881.
The optical system of railway lamps was in great need of improvement, and here progress could certainly have been made, retarded only by questions of economy. The traditional lamp was dismissed as little better than a common stable lamp, its light little concentrated by a cheap bull's-eye lens, and this did indeed describe most railway lamps. The provision of a reflector could increase the illumination by 7 to 8 times, better lenses could be used, and the burner modified to provide the proper air supply for a larger wick, by using a chimney or equivalent.
The rudest bull's-eye lens was not even a converging lens, merely a piece of common sheet glass heated to softness and formed on a round mold. It could be coloured red with copper oxide, or by a slightly different process also involving copper oxide it could be given a green colour. Both colours were murky and variable. The cup of glass could be filled with molten glass to form a plano-convex lens that would converge the light of the flame to some kind of beam. It must be added that there was no possibility of a yellow colour reliably distinguishable from the red and green.
If flint glass was used instead of sheet glass, much better colours and optical properties were available, but at a higher cost. Gold colours flint glass a wonderful red, turning it into ruby glass. Filled with flint glass to make a lens, a ruby glass cup made a good converging red lens, although about half the intensity was lost. Cobalt, similarly, makes an excellent deep blue, but absorbs too much light from the already deficient in blue light to be useful as a signal glass, except for special purposes. Manganese peroxide produces a purple or violet colour that is also dark, but useful. Green was harder to make, and tended to the yellowish. There was again no good distinct yellow, although the lack is not commented upon at the time.
The optical system of an oil lamp must take into account the size of the flame. Since it is not a point, there is no advantage to stigmatic imaging or any exactness of focus. The greatest advantage of electric lamps was the small size of the light source, once tungsten filaments had been introduced. The early incandescent lamps with carbon filaments would have not been much better than lamp flames. Their short life and unreliability meant that electric lamps were not used in railway signalling for many years. In 1881, the newly invented Faure lead-acid battery was predicted to be of used in railway lamps in place of the oil reservoir. Unfortunately, batteries do not have the energy density of fuels, so this never materialized, except in the case of hand lamps with dry batteries and efficient small bulbs.
In 1881, white was still the colour of safety, though remarks are made recognizing the problem of confusion with other white lights, which is the reason green replaced white about fifteen years later. White lights had the great advantage of being much brighter than red or green lights, however.
The segmented lens, now called a Fresnel lens although Fresnel had nothing to do with it, was just coming into use, borrowed from nautical signalling. A cylindrical segmented lens was illustrated in the article, surrounding the flame and converging its light, which then passed through the usual lenses in the lantern. A lantern, by the way, is the technical term for the container for the burner and reservoir, which can be removed for cleaning, filling and trimming. A semaphore lantern was often cylindrical, a head or tail light close to cubical, and a platelayer's lantern brick-shaped with a rounded wagon top.
A lamp with coloured glasses internally mounted so that they could be rotated to show different colours through the outer lenses was patented by Greenwood and Saxby as early as 1854. The same theory was used (unwittingly) in 1920 with the invention of the searchlight signal in the United States, where coloured spectacles mounted on a polarized relay armature selected red, yellow or green light to be projected by a single optical system. A best-seller in 1881 was a hand lantern that could display white, red, or green at the press of a button.
1881 was in a period of Victorian invention mania. One manufacturer of phosphorescent paint proposed that buffer beams and signal posts be painted with phosphorescent paint to render them visible in tunnels or by night. A modern equivalent, much more practical, is reflectorization. Another inventor proposed using Geissler discharge tubes, like neon lights, for signals. Still another proposed gas jets spelling out "safety" and "danger."
Composed by J. B. Calvert
Created 19 September 1999