3.2 Lighting inventions

The Davy lamp is just one example of an inventive solution to a problem. In that case, existing materials were used to provide the answer. In some cases, though, the development or use of new materials is part of the inventive process, and the new material allows the creation of new products. In this section I will look at some lighting inventions that progressed through materials development.

Although flame safety lamps were only of limited applicability outside coal mines, there was a long-felt need for brighter lighting during much of the 19th century. With the growth in manufacturing industry, complex machines needed continuous supervision and maintenance, and illumination by candles or oil lamps was far from adequate (even during daylight hours). Although intense sources of light had been known since Faraday's work on The carbon-arc lamp , they were not readily portable and were not safe or cheap enough for regular use either in industry or in homes.

The carbon-arc lamp

The carbon-arc lamp was one of the first light sources to exploit the discovery of electricity. The device was fairly simple: a very high voltage was generated and applied to two carbon rods. Holding these rods close together produces a spark between them (Figure 39) because a high electric field is produced in the air gap between the ends of the rods. With a powerful electricity source, the spark will be continuous, and can be extremely bright.

Maximise

Figure 39 Principle of the carbon-arc lamp

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Figure 39 is a diagram of a carbon-arc lamp. Two carbon rods are represented by two rectangles with an additional triangular point at one end. These rods are arranged horizontally with their pointed ends facing towards each other but with a noticeable space between the points. Above the two rods a circle is used to represent a high-voltage generator. Two lines are used to represent the electrical connections between the rods and the generator. Starting at the generator, one line leaves the right-hand side of the generator and connects to the right-hand end of the right-hand rod. The other line leaves the left-hand end of the left-hand rod and connects to the left-hand side of the generator. Between the pointed ends of the rods, a zig-zag line is drawn in an arc to represent a bright spark between the pointed ends of the rods. Overall the diagram shows a continuous electrical circuit going from the generator to the right-hand rod, through the right-hand rod, across the gap between the rods, through the left-hand rod and back to the generator.

Figure 39 Principle of the carbon-arc lamp

The problem with this design is that the rods are burnt off rapidly, and so need to be continuously pushed together in order to maintain the electric current and hence the light. If the gap between the rods becomes too large, the electric field will drop to a point where the spark will no longer form.

The development of coal gas – flammable gases extracted from coal – provided a readily available source of energy when piped distribution systems were constructed. However, the illumination from naked flames was not great, being not much better than a candle, and there was the continuing danger from fire. This problem did result in the invention of the incandescent mantle, where the flame is played on a woven structure impregnated with metal salts that shine brightly, i.e. are incandescent, when heated in an open flame. A similar development used lime as the incandescent material (hence the expression 'limelight').

One problem with all these methods is that a high temperature is generated by the light source, which could clearly be a fire hazard. There was no 'safe' lighting system available apart from miners lamps in collieries.

Although the concept of electricity had been understood for some time, the development of electricity-generating systems (pioneered by Edison in the USA) encouraged efforts to develop a light source powered by an electric current. If this could be done successfully, then electric light could be the solution to low intensity (candles, oil lamps) or dangerous (carbon-arc, gas flame) lighting. Although electric lighting works by heating a wire until it becomes hot enough to glow brightly (it becomes incandescent ), the invention of the light bulb neatly side-steps the danger of having exposed high temperatures by enclosing the incandescent source in a glass envelope. The key to the solution of the problem of electric lighting lay not so much in the need for an electric current, since this was already available, but rather in the need to find a material which could withstand:

1. the high temperatures needed to reach incandescence and
2. glow brightly without melting;
3. which would conduct electricity, and
4. which would last for a reasonable time.

The solution was found independently by Edison in the USA and Swan in Britain in about 1879. Heated carbonised filaments would provide a continuous light if they were protected from oxidation (i.e. burning) by being enclosed in either an inert atmosphere (which does not react with the filament) or a vacuum (Figure 40). Both solutions worked, although the lifetime of the lamps was still limited by today's standards. Nowadays, the preferred filament material is tungsten. It possesses a very high melting point (3410 ºC) and can be readily fabricated into the coiled wire filament of electric light bulbs.

Figure 40 An early carbon filament lamp

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Figure 40 is a drawing of a carbon filament lamp. The lamp comprises a glass bulb encasing a carbon filament. The glass bulb is held above a rectangular base by two supports. The bulb is shaped like a straight smooth gherkin. Each support has an electrical connection and is therefore made of a conducting material i.e. metal. The base, shaded differently, must be an insulator e.g. wood. The top support is high enough to hold the bulb above the base and the bottom support is flat underneath the bulb. A wire runs down from the top support, into the bulb and connects to the top of the carbonised filament. Another wire goes up from the bottom support, into the bulb and connects to the bottom of the filament.

Figure 40 An early carbon filament lamp

There is much interest in improving the efficiency of the incandescent lightbulb. One innovation involves halogens such as bromine and iodine. Small amounts added to the atmosphere in the bulb increases the efficiency by allowing the filament to work at a higher temperature, and halogen bulbs are used in car lights as well as in domestic lights. Fluorescent bulbs have been promoted for their greater efficiency and lower working temperature, although more expensive to purchase. Light-emitting diodes (LEDs) have also been developed, being of simple construction and greater efficiency. Domestic light bulbs using LEDs are available, although at some great expense. They are also now widely used in small torches.