IELTS Reading: Sentence completion

For most of human history, city streets fell dark at sunset, and anyone going out at night carried a light of their own. The first attempts at public lighting placed the burden on householders: early in the fifteenth century, London’s residents were ordered to hang a lantern outside their doors on winter evenings. The results were feeble: a candle behind a pane of horn or glass lit little more than the doorway beside it, and the rules were widely ignored.

In the second half of the seventeenth century, Paris began hanging candle lanterns above the middle of its streets, paid for by a special tax, and during the eighteenth century candles gave way to oil lamps, which burned longer and more steadily. In the 1780s the Swiss inventor Aimé Argand produced a burner with a hollow, circular wick that drew air up through its centre and gave a far brighter flame. Even so, an oil-lit street was dim by modern standards, and the lamps needed constant attention: their wicks had to be trimmed, and their oil, often obtained from whales, was expensive.

The real transformation began with gas. In the 1790s the Scottish engineer William Murdoch lit his own house with gas produced by heating coal, and in 1807 Pall Mall in London became one of the first streets anywhere to be lit by gas. Within decades, gas pipes ran beneath the streets of cities across Europe and North America. Gas gave a brighter light than oil and, once the pipes were laid, it was cheaper to run. And because the fuel arrived by pipe, nobody had to refill each lamp by hand.

Lighting the lamps, however, still required people. Every evening a lamplighter walked a set round with a long pole, used to turn on the gas and light it, and returned at dawn to put each lamp out; in a large city, lamplighters formed a small army until clockwork timers slowly took over their work in the twentieth century. Gas lighting also became far brighter in the 1880s and 1890s, when the Austrian chemist Carl Auer von Welsbach developed the gas mantle, a delicate mesh that glowed white when heated by the flame. The mantle extended the life of gas lighting at the very moment that it faced a serious rival.

That rival was electricity. In an arc lamp, a brilliant light is produced by a current leaping across a gap between two carbon rods. Arc lamps appeared on some Paris streets in the late 1870s. The light was so intense that several American cities mounted arc lamps on iron towers tall enough to illuminate many blocks at once; some of these ‘moonlight towers’ still stand in Austin, Texas. But the glare that suited broad avenues was harsh in narrow streets, and the carbon rods burned away and needed frequent replacement.

The incandescent bulb, developed independently by Joseph Swan in England and Thomas Edison in the United States around 1880, gave a softer light suited to ordinary streets. Early bulbs used fragile carbon filaments, but once filaments of tungsten were introduced early in the twentieth century, electric lamps became efficient and reliable enough to replace gas, and over the following decades most cities made the change.

From the 1930s, a new kind of lamp began to change the colour of the night. The low-pressure sodium lamp is remarkably efficient, producing far more light for each unit of electricity than a filament bulb. Its light, however, is an intense orange-yellow under which colours are almost impossible to tell apart: a red car and a blue one look much the same. High-pressure sodium lamps, common from the 1970s, gave a pinkish-gold light in which colours were easier to see, and together the two types gave the world’s cities their familiar orange glow.

Since around 2010, that glow has been fading. Light-emitting diodes, or LEDs, have replaced sodium lamps in city after city, and the arguments for them are strong. They often use around half the electricity of the lamps they replace, can run for many years without attention, and can be dimmed late at night or aimed precisely at the road instead of spilling light into bedroom windows. Their white light also makes colours look natural.

Yet the change has raised new concerns. Many early LED street lamps gave a cold, bluish-white light, and blue light is scattered more readily by the atmosphere, adding to the ‘skyglow’ that hides the stars from most city dwellers. Blue-rich light also disturbs the body’s internal clock and confuses wildlife, from insects drawn to lamps in huge numbers to newly hatched sea turtles that crawl towards lit roads instead of the sea. Many cities now choose warmer LEDs as a compromise. Even the claim that lighting makes people safer is less straightforward than it seems. ‘People have always felt safer in lit streets,’ says Dr Helen Carrow, a historian of urban technology, ‘but the evidence that lighting reduces crime is surprisingly mixed.’ Some studies have recorded falls in crime after lighting was improved; others have found no effect, or suggest that the benefit came from the attention a neighbourhood received rather than from the lamps themselves.