Power cuts force us to rely on more primitive methods of illumination, such as candles, gas or oil-lamps. Candles and flame lamps were in fact the main source of illumination for millennia, up until the late Victorian era. It was many years before electricity was harnessed to provide a new form of lighting both homes and businesses. Why did it take so long to develop the humble light bulb (1879), if electricity had been discovered and studied for more than a century before that date?
It was the invention of the battery by Count Alessandro Volta in 1800 which marked a turning point in understanding and controlling the strange "fluid". For the first time, electricity could be produced by placing two dissimilar metals together in a conducting and corrosive liquid. It is a simple experiment: just stick a rod of copper and one of iron in a lemon, and connect the two together in an external circuit, and you will generate electricity. By connecting such devices together in a "pile", large voltages can be generated at will. Sir Humphrey Davy quickly discovered in 1802 that by putting two carbon electrodes a short distance apart, a continuous spark would jump across the gap and provide illumination. However, a high voltage is needed and the carbon is continually consumed so the rods must be pushed into the arc at a steady rate.
It was Davy who then went on to apply the pile of batteries to decompose various molten compounds, and so discover many new elements such as calcium and magnesium. But he became interested in a problem which was worrying many people in the growing coal mining industry. Large numbers of miners were dying in methane gas explosions at the coalface owing to their use of naked flames for illumination. Phosphorescent sources such as rotting fish were no real substitute! Davy was asked to solve the problem after a particularly severe accident at Felling colliery in 1812 when 92 men and boys were killed by a single explosion. His solution came after study of the simple flame. It is the high temperatures developed by the flame which can ignite methane and so create a disaster in a "gassy" pit. So could some device be put around a flame to prevent such ignition yet without putting the flame itself out by preventing air reaching the flame?
The solution was surprisingly simple: iron gauze. When a flame was completely enclosed by gauze, the lamp could be used safely in a gassy atmosphere (Figure 1). Davy's experiments also showed that there was a minimum size of gap in the gauze needed (about 0.5 mm), but anything larger would cause an explosion.
Figure 1: Davy lamp (left), Marsaut lamp (middle) and modern bonnet lamp (right)
When introduced as a working lamp, it encouraged further expansion of the coal industry, and Davy was hailed for his simple but effective invention. This simple idea encouraged a great expansion of the mining industry, providing coal for steam raising and powering the Industrial Revolution.
That is where the story ends as told in the schoolbooks. However, they neglect to add that the death rate from explosions actually grew after its introduction. The reason was simple: the gauze was fragile and could be damaged easily both mechanically and by rusting. Loss of only one wire connection made the lamp unsafe since the critical hole size doubled. In addition, the low illumination from the flame was reduced further by the gauze. Much effort went into improving Davy's concept, ending with the bonnet lamp, where the bonnet around the gauze protects it from damage (Figure 1). The illumination is very poor even with the modern safe lamp, and it is now only used for testing for gas - mainly in other countries, with the demise of deep-mining in the UK.
Through the Victorian era, there was increasing demand for improved illumination, especially in factories attempting to work a 24 hour day to maximise return on capital invested in machinery. The introduction of coal gas with distribution networks, gave gaslights from an early date. Birmingham streets for example, were gas-lit from 1826. The gas was made by heating coal in retorts, a discovery made by William Murdoch (manager of the Soho works of Boulton and Watt, makers of steam engines), cheap coal being made available by Davy's lamp. The Soho factory was lit by 2600 lights as early as 1802 by Murdoch. A big improvement in lighting power came with the invention of the mantle (1885), where the flame impinged on a woven fibre sphere impregnated with special salts. Such lights are still used widely in developing countries where there is no electricity grid, and produce very strong illumination.
It was however electricity which simulated inventors to develop some way of converting it directly to light. Two such individuals, Joseph Swan in Britain and Edison in the USA found the solution in 1879. Working independently, they found that passing a current through a fine carbon filament could produce light. The filament had to be held in a vacuum to prevent oxidation of the carbon, so they provided an evacuated glass bulb. The discovery of a particular type of carbon (from a bamboo sliver) was found by Edison to give the best performance, and indeed, there is a lit bulb in the headquarters of General Electric which claims to be the only survivor still working continuously since it was first lit well over a century ago! Later, however, screening a range of other candidate materials showed that a very high melting metal, tungsten, actually gave greater luminosity when coiled and coiled again. As with carbon, the luminosity comes from the high temperature caused by the passage of electricity through the fine wire: the finer the wire, the greater the resistance. The same idea is used in the Edison fuse which protects circuits from overloading.
In the late Victorian period, the invention of the light bulb provided a much safer alternative to the open flame of the gaslight, especially in the often hazardous atmospheres of factories. Distribution networks were gradually expanded, the electricity being provided by coal-powered generators. In the 20th century, the gas discharge lamp was invented and most recently, halogen tungsten lamps and LEDs (light emitting diodes). The latter promise much more efficient illumination for the industries of the 21st century.