IELTS Academic Reading Practice Test 198

Few household machines have taken so long to become ordinary. The dishwasher was patented in the United States before the American Civil War, yet a century later most homes still had none, and in Britain the device remained a luxury long after it had become commonplace in North America. The reasons have little to do with the difficulty of building such a machine. They lie instead in plumbing, housing, the price of hot water and the awkward fact that, for most of its history, the dishwasher did not wash dishes particularly well.

The earliest designs were simple mechanical imitations of the human hand. In 1850 a patent was granted to Joel Houghton for a wooden machine in which a hand-turned wheel splashed soapy water over a rack of plates. It was cheap and, by most accounts, ineffective: the water was rarely hot enough, the plates were not separated from one another, and the whole device had to be filled and emptied by hand. Similar inventions followed, but none solved the central problem, which is that removing grease from a plate requires water that is both hot and directed with some force.

The person usually credited with the first practical machine is Josephine Cochrane, a wealthy resident of Shelbyville, Illinois. According to the account she gave, she was annoyed because her servants chipped her fine china while washing it, and she decided that a machine would do less damage. Her design held the dishes in wire racks, separated so that water could reach every surface, and used a pump driven by a belt to spray hot soapy water upwards from a boiler below. She was granted a patent in December 1886. The key insight was not the spray but the rack: by fixing each item in position, she made it possible to direct a jet of water at it reliably.

Cochrane's machines were never intended for ordinary kitchens. They were large, built of copper and brass, and designed for hotels, hospitals and restaurants, where one machine could replace several employees. She showed her equipment at the World's Columbian Exposition in Chicago in 1893, where several restaurants on the site ordered it, and she went on to found a manufacturing company. That business was later acquired by Hobart, an American firm already known for commercial mixers, and the domestic KitchenAid machines that Hobart began to sell in the late 1940s descended from the same line.

The next significant step was taken in Britain. In 1924 William Howard Livens, an engineer better known for his work on weapons during the First World War, patented a machine with the features that now define the appliance: a door in the front that opened downwards, and rotating arms that sprayed water over the dishes from below. Livens's design was recognisably modern, but it still had to be filled from the hot tap by hand and still required a plumber to install it.

Adoption was nevertheless slow, for practical reasons. A dishwasher needs a supply of genuinely hot water, which most households did not have until the middle of the twentieth century, when electric immersion heaters and central-heating boilers became common. It needs space in a kitchen and a permanent connection to the plumbing. Early machines leaked, and detergents that would not foam excessively inside a spinning pump did not yet exist. In the United States, where new suburban houses were being built with larger kitchens, the built-in machine became a selling point during the 1950s. In Britain, where housing was older and smaller, sales stayed modest.

From the 1970s the appliance changed from a mechanical device into an electronic one. Timer switches replaced mechanical cams, and thermostats began to measure the temperature of the water rather than the air around the heating element. Sensors that detect how cloudy the water has become allow a machine to shorten a cycle when the load is lightly soiled. Water softeners built into the machine protect it where the supply is hard, since calcium deposits quickly clog a spray arm. The result is an appliance that now uses a fraction of the water and energy that its predecessors did.

Chemistry mattered as much as engineering. Automatic dishwasher detergents combine alkaline substances, which break down grease, with enzymes that digest starch and protein at the relatively low temperatures of a domestic cycle. For decades these powders contained phosphates, which softened water and improved cleaning but washed into rivers and lakes, where they encouraged algae to grow. Several countries restricted them, and from January 2017 consumer dishwasher detergents sold in the European Union were limited to a very small quantity of phosphorus.

Because a machine recycles a small volume of water through several stages, a full load typically requires little more than ten litres, whereas washing the same number of dishes by hand under a running tap may use several times as much, particularly if the water is left to run while items are scrubbed. The advantage disappears if the machine is half empty, or if the plates are rinsed under a hot tap before they are loaded.

Today the dishwasher is close to universal in North American households and common in northern Europe, although fewer than half of British homes have one. The most recent changes are smaller than the earlier ones: quieter pumps, better insulation, and controls that can be operated from a telephone.

A composite material combines two substances that keep their separate identities but behave, together, better than either does alone. In a carbon-fibre composite the reinforcing element consists of bundles of extremely fine filaments, each only five to ten micrometres in diameter, or about a tenth as thick as a human hair, while the matrix is a polymer, usually an epoxy resin, that encloses them and keeps them in position. The fibres carry the load; the resin transfers it from one fibre to the next and protects them from moisture and abrasion. Neither material would serve on its own, because the fibres are strong but flexible, while the resin is stiff but brittle.

Carbon fibre begins as a precursor, a fibre of another material that is already fine and uniform. Most of the world's supply comes from polyacrylonitrile, or PAN, a synthetic polymer originally developed for textiles; rayon and pitch are also used. The precursor is first stabilised by heating it in air at around 250°C, which rearranges its molecular structure and stops it melting at the temperatures to come. It is then carbonised at 1,000°C or more in an atmosphere of nitrogen or argon, so that everything except carbon is driven off. Up to 95 per cent of the finished fibre is carbon, arranged in sheets of hexagonal rings aligned along the fibre axis.

High-performance carbon fibre appeared in the late 1950s, when researchers in Japan and the United States produced strong fibres from PAN. The process was refined in Britain at the Royal Aircraft Establishment in Farnborough, where in 1963 three scientists patented an improved method of stretching the fibre while it was being carbonised. Stretching aligns the graphitic sheets along the axis, and alignment is what gives the fibre its stiffness. The technique was licensed to manufacturers, and within a decade carbon fibre was being used in aircraft parts and sports equipment.

The fibres are sold as tows — bundles of thousands of individual filaments — and are woven or laid up in the shape the finished part requires. In the commonest manufacturing route, the tow is impregnated with uncured resin and placed onto a mould by a machine that lays each strip in a chosen direction; the assembly is then cured in an autoclave, a pressurised oven in which the resin hardens. Because the fibres can be pointed wherever strength is needed, a component can be thick where loads are concentrated and thin elsewhere, something that cannot be done with a sheet of steel.

The resin is more than a glue. It must bond chemically to the fibres, which are treated with a thin coating, known as a sizing, applied by the manufacturer for exactly this purpose. It must also tolerate the temperatures and chemicals the part will meet in service, and it must cure without shrinking enough to crack. A weak bond between fibre and resin allows a crack to run along the interface, so much of the research on composites is concerned less with the fibre than with the surface chemistry of the boundary between the two materials.

The mechanical properties of the material follow from its internal arrangement. Weight for weight, carbon fibre is several times stronger than steel; a finished composite may have a density of 1.6 grams per cubic centimetre, against 2.7 for aluminium and 7.8 for steel. But the strength is directional. Along the fibres the composite is extraordinarily stiff; across them it depends almost entirely on the resin, which is comparatively weak. Engineers call such behaviour anisotropic, and it explains why a component must be tested in every direction in which it might be loaded.

The largest single source of demand is aerospace. Roughly half the weight of a Boeing 787 and of an Airbus A350 is composite, principally carbon fibre, which allows cabins to be pressurised to a higher humidity and lets wings flex further without cracking. Wind-turbine blades, which now exceed 100 metres in length, are built from glass fibre with carbon fibre in the spar, the internal beam where stiffness matters most. The material also appears in racing cars, bicycles, tennis rackets and fishing rods, and in the pressure vessels that store hydrogen at several hundred times atmospheric pressure.

Carbon-fibre composites have drawbacks that have confined them to applications where their cost can be justified. The fibre is expensive and the curing process slow; an autoclave large enough to take an aircraft wing is a considerable investment. Failure is sudden: unlike metal, which bends and gives warning before it breaks, a composite can shatter without deforming. Impact damage is hard to see, because the surface may look intact while layers beneath it have separated. Drilling holes for rivets cuts the fibres and creates concentrations of stress, so parts are often bonded instead, which makes repair difficult.

Recycling is the material's most obvious weakness. Because the resin in most composites is a thermoset — a polymer that cannot be melted and reshaped — the fibre cannot simply be recovered by heating. Milling scrap yields short, disordered fibres worth little; pyrolysis, in which the resin is burned away in the absence of oxygen, recovers longer fibres but damages their surface. Researchers are therefore testing thermoplastic matrices, which can be remelted, and precursors derived from lignin, a waste product of paper-making. Others are embedding sensors in the layup so that a component can report its own damage.

Walk through the centre of almost any European city and you will pass buildings whose windows stay dark for most of the year but which fill, in summer, with the sound of suitcases on the stairs. These are properties that exist mainly to be let to visitors for a few nights at a time, and they are the visible result of a remarkable change in the way people travel. The companies that arrange such stays are among the most successful businesses of the past two decades. The question I want to consider is whether the cities that host them have on balance gained or lost.

The case in favour is easily stated. Renting out a room gives a household an income that may make the difference between keeping a home and losing it, particularly in a city where wages have not kept pace with housing costs. Visitors spend money in neighbourhoods that have no hotels, so the benefits of tourism spread beyond the centre. Families travelling with children can cook for themselves, and people who cannot afford hotel prices can still take a holiday. The stock of accommodation also expands quickly when demand rises, as it does whenever a city hosts a large event, without anyone having to build anything.

The case against begins with housing. When flats are withdrawn from the long-term market and let to visitors instead, the supply available to residents falls. A study of American cities published in 2021 estimated that a rise of one per cent in the number of short-term listings in a neighbourhood was associated with an increase of roughly 0.018 per cent in rents and 0.026 per cent in house prices — small figures that become substantial when listings multiply across a whole district. The effect is greatest in the most desirable areas, which are exactly the areas where people most want to live.

Residents also bear costs that economics does not capture. A building in which half the flats are let to visitors for two nights at a time has no neighbours in any meaningful sense, only a succession of strangers. Noise, rubbish, insecure front doors and the disappearance of the corner shop in favour of the key safe are the complaints most often made to councils, and they are difficult to measure and almost impossible to compensate. In a few districts the irritation has even become an election issue.

Cities have responded in different ways. Some have banned short lets in the centre outright; others have limited the number of nights a year for which a home may be let; a few have required hosts to register and to display a number in their advertisements. New York took the strictest route. From September 2023, hosts there had to register with the city, and the platforms were made responsible for checking that a registration was valid before accepting a booking. The number of listings in the city fell sharply within months.

Enforcement is the heart of the matter, because rules can only work if the companies that take the bookings cooperate. A registration scheme that relies on hosts to comply voluntarily will be ignored by precisely the operators who cause most of the trouble. This is why the New York approach matters: it placed the burden of verification on the platform, which holds the data and takes a commission on every night sold. European governments have begun to move in the same direction, and in 2024 the European Union adopted rules requiring platforms to share information about their listings with national authorities.

Yet the case for prohibition is weaker than it looks. A ban removes the income that many ordinary households depend on, and it does nothing about the tourists, who simply move to hotels — an industry that is itself concentrated, and whose prices rise when supply is constrained. It also drives activity underground, into informal arrangements that nobody inspects and nobody taxes. Nor did the platforms create the housing shortage; rent controls, planning rules, slow construction and population growth did most of that. Even in the worst-affected cities, the share of the housing stock given over to short lets is usually only a few per cent.

My own view is that the distinction that matters is not between platforms and hotels but between a household letting a spare room, or a home while the family is travelling, and an operator running dozens of flats as an unlicensed hotel. Activity of the first kind is small in scale and harms almost nobody. Regulation should therefore be built around that line. Registration, a cap on nights, a rule that a short let must be the host's main residence, and a requirement that the platforms verify all three would leave the first group alone and make the second much harder to run.

None of this is a matter of technology. The platforms merely made an old practice — taking in lodgers — cheap to arrange at a distance and easy to find. What failed was the rulebook, which had been written on the assumption that a paying guest would always be known to the person who let the room. Cities that have redrawn it, rather than banning the practice outright, appear to be doing best.