IELTS Academic Reading Practice Test 168

Air conditioning is now so ordinary in many parts of the world that its novelty is easily forgotten. For most of history, people in hot climates worked with the climate rather than against it. Houses had thick walls that absorbed the heat of the day slowly, shaded courtyards and small windows. In parts of the Middle East, tall towers caught the breeze and funnelled it over water, and in South Asia woven screens soaked in water cooled the air passing through them. Ice, cut in winter and stored in insulated pits, was a luxury for the rich.

The scientific study of cold began in the eighteenth century. In 1748 William Cullen, a physician at the University of Glasgow, used a pump to create a partial vacuum above a container of ether; the liquid boiled and left a layer of ice. Cullen had no practical application in mind and did not develop the effect. A decade later, Benjamin Franklin and John Hadley, working in Cambridge, experimented with the evaporation of ether and other volatile liquids, and Franklin observed that a thermometer wetted with ether could be cooled far below the freezing point of water.

The idea of a closed cycle, in which the same fluid is repeatedly evaporated and condensed, was described in 1805 by Oliver Evans. Evans never built the machine. The first working vapour-compression refrigerator was constructed in 1834 by Jacob Perkins, an American who had settled in London, and patented there; it used ether and was not a commercial success, but it established the arrangement on which almost all later refrigeration has been based.

Every vapour-compression machine since has worked on the same principle. A volatile liquid, known as the refrigerant, is allowed to evaporate inside a coil, and in doing so it absorbs heat from the space around it. The vapour is then drawn into a compressor, which squeezes it and raises both its pressure and its temperature. In a second coil, exposed to the outside air, the hot vapour gives up its heat and condenses back into a liquid. Finally the liquid passes through an expansion valve, which drops the pressure so that the cycle can begin again.

Others approached the problem from a different direction. John Gorrie, a doctor in the small Florida town of Apalachicola, believed that the fevers that killed so many of his patients were caused by heat and stale air. In the 1840s he built a machine that compressed and expanded air to make ice, which he used to cool the rooms of the sick. He was granted a United States patent in 1851, but the venture failed: the ice trade regarded him as a rival, his backers withdrew, and he died in poverty.

Practical refrigeration came instead from the food trade. James Harrison, a Scottish-born journalist who had moved to Australia, patented an ice-making machine in 1856 and supplied ice to breweries and butchers. In 1859 the French engineer Ferdinand Carré built an absorption machine that used ammonia dissolved in water, and versions of it were soon carried on ships that took chilled meat across the oceans.

In the twentieth century the industry grew out of a manufacturing problem rather than a wish for comfort. Printers and textile makers had found that moisture in the air affected their products, and the Sackett-Wilhelms Lithographing and Publishing Company in Brooklyn could not print a reliable colour image, because paper swelled and shrank as the humidity changed and the colours failed to line up. In 1902 Willis Carrier, a young engineer working for a firm in Buffalo, New York, designed an apparatus that passed air over cold coils. Water vapour condensed on the coils and the air became drier, and as a side effect it became much cooler.

The machines that Carrier built were sold as air washers or as dew-point control apparatus. The phrase 'air conditioning' was coined by Stuart Cramer, a textile engineer in North Carolina, who in 1906 patented a device that added moisture to the air in a mill and used the new term to describe what it did. Only later did the words come to mean the cooling and drying of air for human comfort.

Comfort cooling arrived slowly. In 1914 a wealthy engineer named Charles Gates installed a Carrier system in his mansion in Minneapolis, but domestic equipment remained ruinously expensive for decades. The change came through the cinema. On Memorial Day in 1925 Carrier demonstrated his apparatus at the Rivoli theatre on Broadway and, according to the company's own account, the audience sat through the film twice to enjoy the cool air. Cinema owners quickly discovered that air conditioning attracted summer audiences, and the cold cinema became the most visible sign of the new technology.

Early machines used refrigerants that were dangerous. Ammonia, sulphur dioxide and methyl chloride are toxic, and some are also flammable. In 1928 the chemists Thomas Midgley Jr and Albert Henne, working in a General Motors laboratory directed by Charles Kettering, developed chlorofluorocarbons, marketed under the name Freon, which were stable, non-flammable and apparently harmless. Window-mounted room air conditioners followed in the early 1930s, and after the Second World War cheap mass-produced units transformed daily life in the warmer parts of the United States, helping cities to grow and factories to keep working through the summer.

The environmental cost of that success became clear only later. Chlorofluorocarbons were found to destroy ozone high in the atmosphere, and a phase-out agreed under the Montreal Protocol in 1987 obliged manufacturers to find substitutes. The chemicals that replaced them, hydrofluorocarbons, do no harm to ozone but are powerful greenhouse gases.

Pick up a printed page and it reflects the light around it, holds its image without a battery and can be read in full sunshine or in a dim room. For most of the twentieth century these were precisely the qualities that screens lacked. A liquid-crystal display produces its own light or relies on a backlight, needs power continuously, and becomes difficult to read in bright light. Electronic paper, also called e-paper, was designed to imitate the behaviour of ink on paper rather than the behaviour of a lamp, and it has found a place in reading devices, shop shelves and signs.

Almost all electronic paper sold today is electrophoretic. Each picture element is a cluster of transparent microcapsules, roughly forty micrometres across, scattered through a thin film. Inside each capsule a clear fluid carries white particles of titanium dioxide, which are given a positive charge, and black particles of carbon, which are given a negative one. Apply a voltage and the particles move through the fluid, the white ones towards the viewer or away from them according to the polarity. The colour of the pixel is the colour of whichever particles have reached the surface.

The technology has a longer history than its present visibility suggests. In the 1970s Nick Sheridon and his colleagues at the Xerox research centre in Palo Alto built a display they called Gyricon, in which tiny two-coloured spheres, black on one side and white on the other, were embedded in a sheet of rubber and rotated by an electric field. The idea worked but attracted little commercial interest. In the late 1990s a group at the Massachusetts Institute of Technology led by Joseph Jacobson took a different route, using microcapsules of charged pigment, and the company formed to develop the work gave its name to the material that now dominates the market.

Making a screen from the material is closer to printing than to electronics. The capsules are coated onto a plastic sheet, and behind the sheet lies a backplane: a grid of thin-film transistors that applies a voltage to each picture element separately. Grey is produced by driving the particles only part of the way across the capsule, so that some white and some black remain visible. The controller translates an image into a sequence of voltage pulses, known as a waveform, and the quality of that sequence largely determines how cleanly the picture changes.

Because the display reflects light instead of emitting it, it needs no backlight and is legible in direct sunlight without loss of contrast. It also consumes power only when the image is altered: once the particles are in position they stay there, which is why a reading device can run for weeks on a single charge even though its screen is by far the largest component it has. The screens are thin, light and can be manufactured on flexible plastic.

The limitations follow from the same design. Moving particles through a fluid takes time, so a page refreshes in a fraction of a second rather than instantly, and smooth video is beyond reach. Because the particles do not always settle in exactly the same place, a faint trace of the previous page, known as ghosting, may remain until the screen is cleared. Colour is harder still, since the pigments that make the whitest white are opaque, and stacking three coloured layers reduces the brightness of each.

The first colour e-paper, introduced around 2010, solved the problem crudely by laying red, green and blue filters over a black-and-white screen, which muted every shade. Later versions placed coloured pigments inside the capsules themselves, producing softer colours closer to those of a printed poster. Rival approaches include displays in which a droplet of coloured oil is moved sideways by an electric field, which can switch far faster and give more saturated colour, and mirrors so small that they reflect only one wavelength of light.

Electronic paper has settled where its virtues matter most. E-readers, beginning with the Sony Librie in 2004 and followed by the Amazon Kindle in 2007, remain the best-known use, but the larger market is quieter: labels on supermarket shelves that are updated from a central computer, luggage tags at airports, timetables at bus stops and large signs in shops that would otherwise be reprinted every week. Because such displays hold an image indefinitely, they can run on a small battery for years.

What makes the material convincing is not only its lack of a backlight but the way it scatters light. A sheet of paper reflects light in all directions, so it looks the same from any angle; a glossy screen reflects it in one direction and dims as the viewer moves to the side. The surface of an electronic display is roughened to scatter light in the same way, and the white particles are chosen for their brightness. The result is less crisp than print, but many readers find it easier on the eye than a phone over long periods.

Research now runs in several directions. One is flexibility: prototype screens can be rolled up or stitched into clothing, using plastic backplanes in place of glass. Another is colour, where the aim is to match printed paper without the brightness penalty of filters. A third is speed, since a display that refreshed quickly enough could be used for notes, diagrams and textbooks. Whether electronic paper ever replaces paper for everyday use is doubtful, given how cheap and how good paper already is; the more realistic prospect is that the two coexist.

A letter written in 1850 can still be read by anyone who can read handwriting. The paper may be yellowed and the ink faded, but nothing stands between the reader and the words. A document created on a computer in 1995 is a different matter. The machine that produced it may no longer exist, the program that interpreted it has probably been abandoned, and the disk on which it sits may have lost the magnetic pattern that carried the text. Future historians will know more about the Victorians than the Victorians knew about their own grandparents, and quite possibly less about us.

The problem is often described as if it were a single one, but it is at least three. The medium itself decays: magnetic tape stretches and loses its signal, and the dyes in recordable discs break down. The equipment needed to read the medium disappears, usually within a couple of decades. And the format becomes unreadable, because the software that gave meaning to a file is no longer installed anywhere.

The best-known illustration is the BBC Domesday Project. In 1986 the corporation, working with Acorn and Philips, published an ambitious survey of British life on two interactive videodiscs, compiled with contributions from a million people. The discs were still physically intact in the early 2000s, but the machines that could play them had gone, and the data were recovered only through an emulator written by researchers at Leeds and Michigan. That project was rescued; many others have not been.

Copying is necessary but not sufficient. The most robust idea in the field is captured by an acronym, LOCKSS — lots of copies keep stuff safe — a scheme developed at Stanford in the late 1990s in which libraries hold identical copies of journals and compare them periodically, so that damage to one copy is detected and repaired from another. The Internet Archive, founded in 1996, does something similar on a vast scale, storing snapshots of the web. Neither solves the problem of meaning: a perfect copy of an unreadable file is still unreadable.

Two strategies dominate. Migration converts material into current formats, and it must be repeated every few years; it keeps files usable but risks losing whatever the old format recorded that the new one cannot, and it requires someone to notice what matters before it is converted. Emulation instead recreates the original environment on modern hardware, so that a file can be read as its first users read it. Emulation preserves far more, but it is expensive, it depends on detailed documentation of machines that were never designed to be documented, and it is generally more demanding of resources than migration.

Behind both strategies lies a harder question: what to keep. The volume of digital material is enormous and grows continuously, and storage, although cheap, is not free once it must be checked, described and migrated. Archivists speak of selection as the least comfortable part of their work, since the value of a record is often invisible when the decision is made. Legal deposit has been extended in several countries to cover electronic publications, which removes some of the choice from individual institutions, but legislation cannot preserve what was never collected.

There is a case for calm. Digital copying is exact, so a text can be reproduced a thousand times without a single error creeping in, and search makes an archive usable in ways that a room of boxes never was. Paper has not been reliable either: fires, floods, wars and simple neglect have destroyed countless manuscripts, and the survival of what we do have is partly a matter of luck. Vint Cerf, one of the architects of the internet, warned in 2015 that the coming century might be a digital dark age, but warnings of that kind have been issued before.

Yet the difference in kind is real. A paper archive can be neglected for a century and then read; neglect is precisely what destroys a digital one. Nothing about a hard disk announces that it is failing, and no one notices a missing file until somebody asks for it. Preservation, in other words, is not a product that can be bought once. It is a habit, and habits cost money and attention every year, which is why the records most at risk are often those of organisations that have no archive and no reason to think they need one.

My own view is that the danger lies less in the technology than in the absence of institutions willing to take responsibility for it. Where money, standards and trained staff exist, digital material survives remarkably well; where they do not, no format is safe. What is needed is unglamorous: sustained funding rather than one-off projects, open formats rather than a single company's products, and a willingness to make the same decision again every few years as the technology moves. Individuals can help themselves by keeping several copies in different places and by printing the things they could not bear to lose.

None of this is an argument for nostalgia. The digital record is incomparably richer than anything the nineteenth century produced, and most of it will survive. But it will survive because people choose to keep it. Survival is not a property of the medium; it is a practice.