IELTS Academic Reading Practice Test 196

For most of the twentieth century, the screen was a piece of furniture. Televisions and computer monitors stood on the floor or the desk, and the larger the picture, the deeper and heavier the cabinet became. Inside almost all of them sat the same component: the cathode-ray tube, a glass funnel from which the air has been pumped out, in which a narrow beam of electrons is fired at a surface coated with materials that give out light when struck. The principle is easy to state and was extraordinarily difficult to turn into a reliable product.

The story begins with experiments on electricity in gases. In the middle of the nineteenth century, experimenters such as Julius Plücker and his student Johann Wilhelm Hittorf passed electric currents through sealed glass tubes containing gas at very low pressure. The gas glowed, and so did the glass opposite the negative electrode, suggesting that something was crossing the tube and making the glass fluoresce. In the 1870s William Crookes, working in London, improved the vacuum inside such tubes and studied these rays more closely; they became known as cathode rays.

What the rays actually consisted of remained disputed for decades. Some physicists held that they were a form of light, others that they were a stream of charged particles. In 1895, Wilhelm Röntgen, experimenting with a cathode-ray tube in Würzburg, noticed that a screen well away from the tube glowed even when the tube was wrapped in black card: he had discovered X-rays. Two years later, J. J. Thomson measured how strongly the rays were deflected by electric and magnetic fields and concluded that they were streams of very light, negatively charged particles, soon named electrons.

The person who turned the tube into a practical instrument was Karl Ferdinand Braun. In 1897, in Strasbourg, he fixed a fluorescent screen at the flat end of the tube and used magnetic coils to swing the beam rapidly to and fro. The electrons left a bright trace wherever they struck, and if the deflection was driven by the electrical signal under investigation, the trace reproduced the shape of that signal. Braun's tube became the oscilloscope, an instrument for displaying waveforms that laboratories continued to use a century later; he shared the Nobel Prize in Physics in 1909.

Television followed more slowly. The first person to transmit recognisable moving pictures was John Logie Baird, in London in 1926, but his system was mechanical, using a spinning perforated disc to scan the image. The electronic alternative was developed in the United States by Vladimir Zworykin, a Russian-born engineer who had worked in St Petersburg with Boris Rosing, who had already used a cathode-ray tube as a receiver. Zworykin's camera tube and his picture tube made an all-electronic system possible, and his employer, the Radio Corporation of America, demonstrated it at the New York World's Fair in 1939.

In a modern tube the electrons come from a small metal cylinder, the cathode, which contains a heater. When a current warms the heater, the cathode releases electrons, a process called thermionic emission, and a positive voltage further along the tube draws them forward. A grid between the cathode and the screen controls how many electrons pass and therefore how bright the spot is. Two pairs of coils around the narrow neck create magnetic fields that bend the beam, one pair moving it horizontally and the other vertically; driven in step, they sweep the beam across the screen in a series of horizontal lines. The phosphor goes on glowing for a fraction of a second after the beam moves on, so the eye sees a steady image rather than a travelling dot.

Colour required a more elaborate arrangement. A colour tube has three cathodes, one for each primary colour, and its screen carries not a uniform coating but a pattern of minute phosphor dots grouped in threes, each group containing a red, a green and a blue. A thin perforated metal sheet, the shadow mask, sits just behind the screen; because the three beams arrive at slightly different angles, the mask ensures that each reaches only dots of its own colour. But the mask absorbs a large share of the electrons, so colour tubes had to drive their beams harder. Colour broadcasting began in the United States in 1954, yet receivers were so costly that monochrome remained the norm for a decade.

The envelope was an engineering problem in itself. Atmospheric pressure amounts to about ten newtons on every square centimetre, so the glass had to be thick, the faceplate curved and large tubes bound with a steel rim to prevent an implosion if the glass broke. The glass also contained lead, which absorbed the X-rays that high-voltage electrons produced when they struck the screen.

Television was not the only employer of the tube. Oscilloscopes displayed electrical waveforms in laboratories and workshops; radar operators watched echoes on cathode-ray screens during the Second World War; and the first stored-program computers used a modified tube, the Williams tube, as memory. Medical and industrial monitors relied on it too.

Flat panels ended its dominance. Liquid-crystal displays were thinner and lighter, and once manufacturing improved they became cheaper at large screen sizes, while plasma displays took the market for very large sets. By the late 2000s most manufacturers had stopped making cathode-ray tubes. Factories closed, and a device that had shaped the appearance of the twentieth century became an object for museums and recycling centres.

The energy in a wave began as sunlight. Sunlight warms the air unevenly; the warm air rises, and the resulting winds drag across the surface of the sea, transferring energy to the water. Once set in motion, a swell can travel for thousands of kilometres with remarkably little loss, so that the waves arriving on a beach may have been raised by a storm whose centre has long since moved on. The power a wave carries depends on its geometry: it rises with the square of the height and in direct proportion to the interval between crests.

Wave energy is often confused with tidal energy, though the two are quite different. Tides are produced by the gravitational pull of the moon and the sun and can be predicted decades ahead; waves are produced by wind and can be forecast with confidence only a few days in advance. Even a few days of warning is commercially valuable, because the electricity system must balance supply and demand minute by minute, and a source whose output can be foreseen several days ahead is easier to accommodate. The strongest, most consistent waves are found on exposed coasts in high latitudes, far from many of the cities that need the power.

Engineers have proposed hundreds of devices and built dozens. Most fall into three broad families, according to how they intercept the waves. Point absorbers are small floating bodies that bob up and down. Attenuators are long, hinged structures that lie parallel to the direction of travel and flex as each wave passes. Terminators, a category that includes overtopping devices and oscillating water columns, face the waves head-on. Converters may also be classified by where they stand: on the shore, just offshore, or far out in deep water, where the resource is greatest but the engineering is hardest.

The oscillating water column is the oldest design. A concrete or steel chamber, open to the sea below the waterline, traps a volume of air above the water surface inside it. As a wave arrives, the water level in the chamber rises and compresses the air; as it falls, the air expands again. The moving air is ducted through a turbine, but because the airflow reverses with every wave, a conventional turbine would be idle for half the time. The solution, invented by Alan Wells in the 1970s, is a turbine whose blades are symmetrical, so that it spins the same way regardless of which way the air is travelling. A 500-kilowatt column built into the shoreline on the Scottish island of Islay in 2000 was among the first to supply electricity to the grid.

Overtopping devices work like a hydroelectric scheme in miniature. Waves run up a sloping ramp and spill into a reservoir above sea level; the water then returns to the sea through low-head turbines, driven by the difference in level. Point absorbers are usually the smallest devices, and many can be packed into an array. Each float is connected to a fixed central column, or spar, and its up-and-down motion drives hydraulic rams or a linear generator. Because the float follows the rising and falling water, a well-designed device can extract energy from waves of several different sizes.

None of this has yet produced a large industry. The best-known attempt was Pelamis, a long articulated attenuator developed in Edinburgh, whose hinged segments were pushed against one another by the passing waves. A prototype tested at the European Marine Energy Centre in Orkney in 2004 was the first wave machine to send electricity to a grid from the open sea. Three full-size machines were then installed off the coast of Portugal in 2008, in what was described at the time as the world's first wave farm. Within months the project had been suspended: technical problems had emerged and a financial backer had collapsed. The company went into administration in 2014, having built no further machines.

The obstacles are formidable. A machine must survive the largest waves that arrive perhaps once in fifty years, which may carry a hundred times the power of the average sea state, yet it must also be cheap enough to compete. Salt water causes rapid corrosion of steel, marine organisms colonise every surface, and mooring lines are worked back and forth millions of times until the metal fatigues. Output is also awkward to measure: a device rated at 750 kilowatts may average only a quarter of that over a year, and because the system must pay for capacity available when it is needed, a low capacity factor pushes up the price of every unit generated. Wave power therefore remains several times more expensive than offshore wind.

Research has accordingly concentrated on survival, control and cost. Modern devices carry sensors and software that adjust the force with which the float resists the water, so that it resonates with the incoming waves and extracts more energy; the same controls can be slackened when a storm is forecast. Developers test their machines at the European Marine Energy Centre in Orkney, which offers grid-connected berths with moorings and cables, and at similar sites elsewhere. Several companies are now designing hybrid platforms that pair a wave device with a wind turbine, sharing a mooring and an export cable, on the reasoning that wind and waves rarely peak at the same moment.

The electronic tag has become one of the quietest and most widespread instruments of punishment in the modern state. In England and Wales alone, tens of thousands of people are monitored at any one time, some confined to their homes for part of the day, others tracked by satellite wherever they go. The device is rarely mentioned in public debate, perhaps because it lacks the drama of a prison, yet it raises a question that goes to the heart of penal policy: whether the state should punish people by taking away their liberty, or merely by watching them. I believe the tag is defensible, but only in a narrow set of circumstances, and it is being used far beyond them.

In the 1960s a group of psychologists in the United States built a wearable transmitter and tested it on young offenders, hoping that it would allow them to live at home while remaining in contact with a supervisor; the equipment was too unreliable to be practical, and the experiment was largely forgotten. The first operational scheme came in 1983, when a judge in Albuquerque, New Mexico, sentenced an offender to house arrest with an electronic monitor. According to the accounts that circulated at the time, the judge had been struck by a comic strip in which a villain was tracked by a device strapped to his wrist.

From that unlikely beginning the practice spread quickly. England and Wales ran pilot schemes in 1989, introduced curfew orders in the 1990s and extended monitoring across the country at the end of that decade; the United States, Australia, Canada and many European countries developed their own programmes, and by the 2000s the tag had become a standard sentencing option. It is now used at almost every stage of the system: as a condition of bail, as a sentence in its own right, and as a requirement of release from prison.

Two technologies do most of the work. A radio-frequency tag is a transmitter strapped to the ankle that communicates with a receiver installed in the offender's home; the monitoring centre knows whether the wearer is in the house during the hours required. A satellite tag contains a positioning receiver and a mobile-phone connection, and records the wearer's whereabouts continuously, so that the authorities can follow a route or set areas that the wearer must avoid. Both generate a stream of data, such as a curfew breach or a failing battery, which staff in a control room must interpret.

The argument for monitoring is at its strongest when it replaces prison. What matters more than money is what the tag preserves. A person who keeps a job, a home and contact with children is far less likely to return to crime than one who loses all three, and a short prison sentence is a reliable way of destroying the first two. Used as an alternative to a sentence of a few weeks, the tag looks like a rare policy that saves money and does less damage at the same time. Despite this, the number of people in prison has not fallen.

How much good it does is less certain. Compliance with curfew orders is high: most monitored offenders observe their conditions, and breaches are usually dealt with quickly. But compliance is not reform. The evidence that monitoring reduces reoffending is thinner than its supporters imply. Government evaluations of early release with a tag have generally found that those released under a curfew were no more likely to reoffend than comparable prisoners who stayed inside, which is a modest finding, since it suggests that the tag does no harm rather than that it reforms anyone. Randomised studies in several countries have produced mixed results, and the more rigorous the design, the smaller the effect tends to look.

Three concerns deserve more attention than they receive. The first is net widening. When a new sanction is introduced, it is seldom used only for the people it was designed for; it is added to the array of options, and the total number under supervision grows. A measure intended to keep minor offenders out of prison ends up placing more of them under state control. The second is duration. A curfew of a few weeks is a restriction; a curfew of two years, enforced by an ankle unit that must be charged daily, is a form of imprisonment in the home, and it imprisons the family too. Partners and children live with the receiver, the inspections and the knowledge that one of them is being watched. The third is competence. In 2013 an audit revealed that two of the largest contractors had been charging the government for tags that were not fitted or not working, and for monitoring people who were in prison or had died; both companies eventually repaid millions of pounds.

None of this leads me to reject the technology. A tag that substitutes for custody, that is time-limited, that is supervised by officials who can see the data and that is backed by support such as housing and treatment can be both cheaper and more humane than a cell. What I object to is the tag used as an addition: an extra punishment bolted onto a sentence because it is available, or a means of extending surveillance over people who would otherwise have been left alone. The device itself is neutral: it measures whereabouts and nothing else. Whether it reduces suffering or increases it depends entirely on the rules we write around it, and on whether those rules rest on evidence rather than on political convenience.