IELTS Academic Reading Practice Test 145

The idea that falling water can do useful work is ancient. For more than two thousand years, water wheels turned by rivers and streams have ground grain, sawed timber and lifted water for irrigation. The principle behind modern hydropower is essentially the same. Water stored at a height possesses potential energy; when it is allowed to fall or flow downhill, that energy becomes kinetic energy, and a machine placed in its path can capture it and turn it into movement. The crucial difference in the modern era is that the turning shaft is connected to a generator, which converts mechanical energy into electricity. The faster and more steadily the water falls, the more power can be extracted.

The first known use of water power to generate electricity occurred in 1878 at Cragside, the Northumberland home of the industrialist William Armstrong. Armstrong, who had made his fortune from engineering and armaments, was an enthusiastic adopter of new technology. He dammed a small lake on his estate and piped water down to a turbine that drove a dynamo, and the electricity produced lit a single arc lamp in his picture gallery. It was a private demonstration rather than a public service, but it showed that falling water could produce electric light.

Three years later, in 1881, the town of Godalming in Surrey began a public electricity supply powered by water. A dynamo at a local mill was turned by the River Wey, and the current was carried along streets to illuminate shops and houses. The experiment was short-lived, because the town soon switched to a steam engine, but it demonstrated that water-powered generation could serve a community.

Across the Atlantic, the first hydroelectric central station in the United States opened in 1882 at Appleton, Wisconsin. Built by H. F. Rogers on the Fox River, the Vulcan Street plant had a capacity of about 12.5 kilowatts. It supplied a paper mill and a few nearby houses, and it was soon followed by many small stations attached to mills and factories. These early plants used direct current, which could be transmitted only a short distance.

The limitations of direct current were overcome by alternating current, which can be transformed to high voltages for transmission and then reduced again for use. The most famous demonstration of the new technology took place at Niagara Falls. In 1895 the Adams Power Plant began operating, designed by engineers working with George Westinghouse and using the polyphase alternating-current system developed by Nikola Tesla. The following year its power was sent to the city of Buffalo, more than thirty kilometres away. The success at Niagara convinced utilities that large hydroelectric stations could supply distant cities.

As stations grew, engineers developed turbines suited to different conditions. The Pelton wheel, patented in the 1880s, uses a jet of water fired at bucket-shaped blades and works best with a very high fall. The Francis turbine, named after the American engineer James B. Francis, is a reaction turbine in which water flows through curved blades enclosed in a casing; it is the most common design for medium heads. The Kaplan turbine, developed in the early twentieth century, has adjustable blades and is used where the fall is low but the volume of water is large. Careful matching of turbine to site allows efficiency of more than 90 per cent.

The twentieth century brought ever larger dams. The Hoover Dam, completed in 1936 on the Colorado River, was a concrete arch structure that impounded a vast reservoir and eventually housed generators with a combined capacity of more than two thousand megawatts. The dam's generators were among the largest of their time. In the decades that followed, governments around the world treated big dams as symbols of modernisation, combining electricity generation with flood control and irrigation.

A different kind of plant, pumped storage, became important as electricity systems grew. In a conventional hydroelectric station, water flows once from a reservoir through the turbines and away downstream. A pumped-storage plant has two reservoirs at different heights. When demand for electricity is low, cheap power from other sources is used to pump water uphill to the upper reservoir. When demand rises, the water is released back down through the same turbines. The plant therefore acts like a giant battery, storing energy rather than generating it from a river.

The scale of modern hydropower is remarkable. The Itaipu dam on the border of Brazil and Paraguay, which began generating in 1984, has an installed capacity of 14,000 megawatts. The Three Gorges dam in China, fully operational in 2012, is larger still, with 22,500 megawatts, and is the biggest power station of any kind in the world. Hydropower supplies roughly 15 per cent of the world's electricity, and in some countries, including Norway, Brazil and Canada, it provides the majority of power.

Yet hydroelectricity is not without costs. Large reservoirs flood land, displace communities and alter river ecosystems, and decaying vegetation in tropical reservoirs can release methane, a potent greenhouse gas. Output also varies with rainfall, so a drought can leave turbines idle. Modern projects attempt to reduce these effects with fish ladders, minimum flows and smaller, run-of-river designs that divert only part of a stream. The technology that began with a single lamp in a Northumberland gallery is now central to the debate about how the world can generate electricity without burning fuel.

When people imagine the internet, they usually picture satellites and radio waves. In reality, almost all international data travels through cables lying on the ocean floor. These submarine cables carry more than 99 per cent of the traffic between continents, from bank transfers and video calls to the messages that coordinate global shipping. Satellites remain essential for remote regions, ships and broadcasting, but their capacity is limited and their signals are delayed by the distance they must travel. A single modern cable can carry tens of terabits per second, a capacity that no satellite comes close to matching. They are, in effect, the arteries of the global economy.

The first submarine cables carried telegraph signals rather than telephone calls or computer data. A cable laid across the English Channel in 1850 carried messages between Dover and Calais, and a transatlantic cable followed in 1858. That cable failed after a few weeks, and it was not until 1866 that a durable link between Europe and North America was established. Telephone traffic joined the routes in the mid-twentieth century, and in 1988 the first transatlantic fibre-optic cable, TAT-8, entered service, replacing copper conductors with hair-thin strands of glass.

A fibre consists of a glass core surrounded by a cladding of slightly lower refractive index. Light travelling along the core strikes the boundary at a shallow angle and is reflected back into the glass rather than escaping. This phenomenon, known as total internal reflection, allows a signal to follow a curved fibre for many kilometres. The glass is extraordinarily pure, but even so the light weakens as it travels, because tiny imperfections scatter it and the material itself absorbs a little of the energy. The pulses also spread out, blurring the boundaries between one and zero.

To counteract this loss, engineers place repeaters along the cable at intervals of roughly fifty to one hundred kilometres. Early systems converted the light into an electrical signal, amplified it and turned it back into light. Modern cables use optical amplifiers, especially erbium-doped fibre amplifiers, which boost the signal without converting it. A short section of fibre containing erbium atoms is pumped with laser light, and passing pulses stimulate the atoms to release more light of the same wavelength. The signal therefore remains optical from one end of the ocean to the other.

Capacity is multiplied by sending many signals at once. In a technique called wavelength-division multiplexing, several laser beams of different colours are combined and travel through the same fibre. Each colour carries its own stream of data, and at the receiving end prisms or filters separate them again. Early systems used a handful of wavelengths; modern equipment may use more than a hundred. Sophisticated receivers can also disentangle signals that overlap, and digital processing corrects errors introduced by noise and distortion. In this way a single pair of fibres, one for each direction, can carry an enormous volume of traffic.

A deep-sea cable is surprisingly slender, typically about the diameter of a garden hose. At its centre are optical fibres, protected by a gel that cushions them against pressure. Around them lie steel wires that give the cable its strength, with extra armour in shallow water, and a copper conductor that supplies power to the repeaters. The whole assembly is wrapped in tough polyethylene insulation. Near the shore, where the risk of damage is greatest, the cable is buried beneath the seabed by a plough or a remotely operated vehicle; in the deep ocean it is simply laid on the bottom, where it is less likely to be disturbed.

Laying a cable is a lengthy logistical operation. Survey ships first map the route, avoiding underwater volcanoes, steep canyons and areas where fishing is intense. A specialised cable ship then pays out the cable from huge tanks, controlling the tension so that it does not snap or loop on the seabed. In shallow sections, a plough may cut a trench and bury the cable behind the ship. Branching units allow one cable to serve several countries, and landing stations on shore connect it to national networks. Permits must be obtained from every country whose waters are crossed, a process that can take years.

No system is immune to failure. Most damage occurs in shallow water, where anchors and fishing gear drag across the seabed; earthquakes, underwater landslides and even burrowing animals can also cause faults. When a cable breaks, the operator sends a repair ship with a grapnel that hooks the severed end and lifts it to the surface. The damaged section is spliced out and a new piece joined in, after which the cable is lowered back to the bottom. Because a single break can affect millions of users, companies route traffic through several cables and keep spare capacity available.

Submarine cables have limits. They are expensive to build and repair, they require international cooperation, and they cannot easily reach small islands or remote coasts. Radio and satellite links therefore remain important in such places. Nevertheless, demand for capacity continues to grow as more devices and services depend on the internet. New cables now use more fibre pairs and more efficient amplifiers, and some are designed to connect data centres directly. Several projects are also exploring routes through the Arctic, where retreating ice may permit shorter paths between Europe and Asia. The network is not a single system but a dense web of routes, any one of which can fail without cutting off the world.

Few technologies have changed intimate life as quickly as the dating app. A generation ago, most people met partners through friends, work, school or chance encounters. Today a significant proportion of new couples meet online. In the United States, surveys by the Pew Research Center have found that around three in ten adults have used a dating site or app, and the figure is considerably higher among those under thirty. The basic mechanism is simple: users create a profile, browse photographs and short descriptions, and indicate interest by swiping or tapping. If two people express interest, they can exchange messages and decide whether to meet.

The advantages are real. Apps widen the pool of possible partners far beyond the neighbourhood or the office, which is particularly valuable for people in rural areas, for those who have moved to a new city, and for older adults. They can also help members of minority groups whose potential partners are thinly scattered. For people with demanding jobs, browsing profiles is more efficient than hoping for a chance meeting. And for anyone who finds face-to-face approaches difficult, the first contact can be made in writing, without the fear of immediate embarrassment.

Yet the design of many apps encourages a particular attitude. Profiles appear as a stream of cards to be accepted or rejected in a second, and the supply seems endless. Psychologists call this choice overload. In a well-known experiment, shoppers were offered a display of either six or twenty-four kinds of jam. The larger display attracted more interest, but only about 3 per cent of those who saw it bought a jar, compared with around 30 per cent of those who saw the smaller one. Too many options can make a decision feel less urgent.

Apps also promise to find compatible partners through mathematics. The systems are usually secret, but they generally combine information from a user's profile with patterns of previous behaviour, such as which profiles are liked or skipped. In practice, stated preferences are a poor guide to attraction, and the data available to a platform are limited. Psychologists who have studied online dating have argued that no algorithm has been shown to predict the success of a relationship better than chance, although a large number of users does help people to meet. A match is an introduction, not a guarantee.

Money shapes the experience. Most apps are free to download but earn revenue from subscriptions, advertising and one-off purchases that make a profile more visible. This creates an incentive to keep users engaged rather than to find them a partner quickly. Features such as streaks, notifications and limited-time boosts borrow techniques from games and social media. None of this is necessarily dishonest, but the aims of the company and the aims of the user are not identical. A service that keeps people searching earns a steady income.

Safety is another concern. Although most users behave reasonably, harassment, deception and fraud are common enough to require serious attention. Fake profiles can be used to build trust and then borrow money, and people who meet strangers sometimes take risks they would not take otherwise. Companies have introduced photo verification, reporting tools and emergency buttons. Privacy is equally important: profiles can reveal sexual orientation, religion or health status, and a data breach could expose users to discrimination. Regulation has begun to catch up, but it varies widely between countries.

Critics also argue that the endless supply of new faces undermines commitment. If another possibility is always one swipe away, small difficulties may seem easier to escape than to resolve. The evidence for this is mixed. People who use apps are not a random sample of the population, and those already reluctant to commit may be drawn to them. What can be said is that the medium rewards a certain kind of attention: rapid, visual and comparative. Whether that changes the deeper habits of love remains unclear.

It is also worth remembering that mediated romance is not new. Matchmakers, personal advertisements, lonely-hearts clubs and introductions by relatives have long helped people to meet. What is new is the scale, speed and automation of the process. A newspaper advertisement reached a few thousand readers; an app can present thousands of profiles in an evening. Partner search has become more accessible, but also more measurable and optimisable.

In my view, the technology itself is neither the problem nor the solution. Dating apps reflect the society that builds and uses them: impatient, commercial, often generous and often anxious. They offer genuine opportunities to people who would otherwise be isolated, and they can encourage a superficial approach to others. The sensible response is therefore not to reject them but to demand better design and clearer rules. Algorithms should be explained in plain language, safety features should be easy to find, and subscriptions should not reward endless searching. Digital literacy matters as much as regulation.

None of this will abolish the awkwardness of meeting someone new. Relationships succeed or fail because of how people treat each other, not because of the software that introduced them. But the software sets the conditions in which those relationships begin. A dating app can be a useful servant or a demanding master, and which it becomes depends on the choices made by designers, regulators and users themselves.