IELTS Academic Reading Practice Test 204

A thermostat holds a room or a machine at a chosen temperature by switching heating or cooling on and off. It does not measure warmth on an absolute scale so much as notice that the temperature has drifted away from a set point. The sensing element may be a strip of two metals that bends as it warms, a sealed bulb of liquid or gas that expands, or a solid-state device whose electrical resistance changes with heat. Each of these converts a physical change into movement or a signal, and a switch then acts on it. The idea is simple; making it reliable took several centuries.

The earliest recognisable thermostat was built more than four centuries ago by Cornelius Drebbel, a Dutch inventor who spent much of his career in England. Drebbel designed an oven for hatching eggs whose temperature, his admirers claimed, stayed within about a degree of the value he chose. The exact mechanism is uncertain, but it appears to have depended on a vessel of liquid or gas that expanded as it warmed, moving a lever that opened or closed a flap above the fire. Historians of engineering often cite the oven as an early example of feedback control, in which the output of a system is used to adjust its own input.

The word 'thermostat' entered English in the nineteenth century, formed from the Greek thermos, meaning hot, and statos, meaning standing. By then, inventors had begun to experiment with pairs of metals. A bimetallic strip is made by bonding two metals with different rates of expansion; as it warms, the strip curves towards the metal that expands less.

Such a strip can be arranged to tilt a small container of mercury, a liquid metal that conducts electricity. When the strip bends far enough, the mercury rolls to the other end of the container and completes or breaks an electrical circuit, switching a heater or a fan. For much of the twentieth century this elegant mechanism was the basis of domestic thermostats.

The first electric thermostat is usually credited to Albert Butz, a Swiss-born inventor working in Minnesota. In 1885 he patented a device he called the damper flapper, which opened and closed the damper of a coal furnace in response to changes in temperature. When the building grew cold, an electric motor opened the damper to admit more air and make the fire burn faster; when the temperature rose, the damper closed. Butz was not the first person to regulate heat automatically, but his use of electricity to move the damper allowed a small sensor to control a much larger source of heat. The company he founded eventually became Honeywell.

Meanwhile Warren Johnson, a teacher in Wisconsin, had been wrestling with a practical problem: the classrooms in his school were either too hot or too cold, and the janitor who adjusted the furnace had no way of knowing the temperature in each room. In 1883 Johnson patented an electric thermostat that used a thermometer to make and break a circuit, and he went on to build a system that could control the heating of a whole building from one point. His company, later Johnson Controls, installed such systems in schools and public buildings across the United States.

For most of the twentieth century, the domestic thermostat was a simple and durable object. A bimetallic coil sat behind a dial, and a glass capsule containing a bead of mercury acted as the switch. Because mercury is toxic, and because the devices were accurate enough only to within a degree or two, they have largely disappeared from new installations. Their replacement, the thermistor, is a small ceramic component whose resistance falls sharply as it warms. A circuit measures that resistance and decides whether to call for heat.

Electronic controls made possible features that mechanical devices could not offer. A programmable thermostat lets a household set different temperatures for different times of day, so that the heating can be reduced while everyone is asleep or out at work. The first programmable models appeared in the 1980s, but many owners found the instructions too complicated and never used the schedules they had entered. Studies of energy use found that the savings promised by manufacturers were often not achieved in practice, because the devices were poorly set up.

The smart thermostat, which arrived around 2011, attempted to solve that problem by removing the need for programming. It observes how often the heating runs, how quickly rooms warm up and when the occupants are usually at home, and it builds a model of the house and its routine. Some models detect that nobody is in the building and reduce the temperature automatically. Others use weather forecasts to anticipate a cold night. The promise is convenience and lower bills, though independent trials have reported savings that vary widely from one household to another.

Whatever the technology, every thermostat faces a basic conflict. If it switches the heat on and off at exactly the set point, it will do so constantly, wearing out the equipment and wasting energy. Designers therefore build in a deliberate gap, known as hysteresis, so that the heating comes on a little below the set point and goes off a little above it. The size of that gap, and the speed with which the sensor responds, determine how steady the room feels. A good thermostat is not simply accurate; it is comfortable.

An underwater drone is a vehicle that operates beneath the surface without a crew. Engineers divide them into two broad families. A remotely operated vehicle, or ROV, is linked to a ship by a cable that carries power and commands, and it is flown by a pilot watching video screens. An autonomous underwater vehicle, or AUV, carries its own power and follows a mission programmed in advance, though some modern vehicles can be redirected while submerged. ROVs tend to be heavy and powerful; AUVs tend to be slender and independent. The boundary between the two is blurring as batteries improve, and the choice depends on the task and on how much the operator is prepared to spend.

Most underwater drones are built around a pressure hull, a strong tube that protects electronics from the crushing weight of water. At a depth of 3,000 metres the pressure is about 300 times atmospheric pressure, enough to collapse an unprotected box. Buoyancy is managed carefully: tanks of air or blocks of foam give the vehicle roughly neutral buoyancy so that it floats without effort. Propellers or thrusters then provide movement in any direction, and fins may add stability. Because water is about 800 times denser than air, a drone moves slowly; a typical survey speed is two to four knots, and fast motion costs a great deal of energy.

The hardest problem is communication. The radio waves that carry Wi-Fi and telephone signals so easily through air die away within a few metres of seawater, which rules them out for a submerged drone. Instead it sends data by sound. An acoustic modem converts digital information into pulses of sound and back again, and these pulses can travel for several kilometres. The difficulty is bandwidth: sound waves are slow and the frequency range available is narrow, so a link may carry only a few thousand bits per second, less than an old telephone modem. Sending a single photograph can take minutes, and video is impossible. The delay is also noticeable, since sound takes about two-thirds of a second to travel a kilometre.

Navigation is equally demanding. A satellite positioning signal cannot reach below the surface, and the vehicle cannot see landmarks in muddy water. AUVs therefore combine several imperfect sources of information. Inertial sensors measure acceleration. A computer integrates these measurements to estimate position, a method known as dead reckoning. Small errors accumulate, so the vehicle may also use a Doppler log, which measures speed relative to the seabed by bouncing sound off it, or acoustic beacons placed on the sea floor. When it surfaces, it can correct its position with satellite signals. Good navigation is essential for mapping, because sonar data are worthless if the machine does not know where it was when it recorded them.

Power limits almost everything. Most drones run on lithium-ion batteries, and a large survey AUV may work for a day or two before it must be recovered. A different design, the underwater glider, achieves far longer missions by using almost no propulsive power. It changes its buoyancy by pumping oil between an internal bladder and an external one, and small wings convert the resulting slow rise and fall into forward motion. A glider can cross an ocean over weeks or months, but it travels slowly and cannot fight a strong current. Engineers are also testing fuel cells and, in some cases, lithium-sulphur batteries to extend endurance.

The uses of these vehicles are expanding quickly. Survey companies use multibeam sonar mounted on AUVs to map the seabed in high resolution, work that is vital for laying cables and pipelines and for understanding underwater geology. Energy firms inspect the legs of oil platforms and the lengths of pipe on the sea floor with ROVs, avoiding the cost and danger of sending divers to depths where they could not safely work. Marine biologists attach cameras and sensors to gliders to study the chemistry and life of the open ocean. Navies use small drones to search for mines, and rescue teams use them to inspect wrecks and search for bodies.

None of this is cheap or simple. A work-class ROV with its control cabin and winch can cost millions of dollars, and the ship needed to launch it costs more. AUVs are smaller but still expensive, and a lost vehicle represents a serious financial blow. Strong currents sweep a light drone off course, and fishing nets and seaweed can foul propellers. The data themselves are a burden: a single survey may produce terabytes of sonar imagery that must be processed by specialists. Salt water corrodes metal parts, and every seal is a potential point of failure.

Current research is directed at making drones more independent and more cooperative. One line of work involves docking stations on the seabed where a vehicle can recharge its batteries and upload data without returning to the surface, potentially allowing months of continuous operation. Another involves swarms of small, cheap drones that communicate with each other and cover an area together, a principle borrowed from shoals of fish. Improved acoustic links would let a human supervisor guide an AUV in real time from a ship. The long-term aim is a vehicle that can decide for itself what is interesting and investigate it.

On two mornings in July 2021, the argument about space tourism stopped being theoretical. A Virgin Galactic aircraft carrying Richard Branson reached the edge of space and returned; nine days later a Blue Origin capsule took Jeff Bezos and three companions above the same boundary. Neither flight completed an orbit, and neither lasted more than a few minutes. Yet both were presented as the beginning of a new industry in which private companies, rather than governments, would send paying passengers to space. The claims deserve scrutiny.

It is useful to distinguish two kinds of trip. A suborbital flight follows a steep arc, giving passengers a few minutes of weightlessness and a view of the curve of the Earth before the vehicle falls back into the atmosphere. An orbital flight must reach a speed of about 28,000 kilometres an hour, roughly eight times faster, and it requires far more energy, a larger rocket and a heat shield capable of surviving re-entry. Suborbital tourism is therefore cheaper and simpler; orbital tourism remains, for now, a pursuit of the very wealthy. The difference in energy is not a detail. It determines cost, emissions and risk.

The idea is not new. In 2004 a small rocket plane called SpaceShipOne, funded by the Microsoft co-founder Paul Allen, made three flights above the recognised boundary of space and won the Ansari X Prize, a competition designed to encourage private human spaceflight. Virgin Galactic was founded to commercialise the technology, and Blue Origin and SpaceX pursued reusable rockets. Private individuals had already paid to visit the International Space Station, beginning with Dennis Tito in 2001, but those trips relied on Russian government vehicles. What changed in the 2020s was that private companies were now selling the whole experience.

Supporters make three arguments. The first is inspiration: seeing people fly to space, they say, encourages young people to study science and engineering. The second is innovation. Competition between companies has driven the development of reusable rockets, which have lowered the cost of launching satellites and made some scientific missions affordable. The third is freedom: enthusiasts argue that individuals should be allowed to spend their own money as they wish, and that a private industry will eventually make space access routine. These are serious claims and should not be dismissed.

Some of the evidence, however, is weaker than the rhetoric. The claim that spaceflight produces a lasting change in perspective, often called the overview effect, rests largely on the testimony of a small number of astronauts. Studies of the few people who have travelled to space report that they value the experience, but such a select group can hardly be representative. The inspiration argument is similarly difficult to test: science enrolment depends on many factors, and no study has shown that watching a celebrity launch causes more students to choose physics.

The environmental costs are easier to identify. Rockets release gases and particles directly into the upper atmosphere, where they linger far longer than they would near the ground. Black carbon, or soot, from kerosene-fuelled engines is particularly effective at absorbing sunlight and warming the stratosphere, and scientists have warned that a rapid increase in launches could have a disproportionate effect on climate. At present the number of rocket flights is tiny compared with commercial aviation, and the industry's emissions are small. The concern is growth: space tourism is designed to be repeated, and because the exhaust is released high in the atmosphere, each additional flight matters more than its modest quantity of fuel would suggest.

Safety raises a different set of questions. Spaceflight remains a dangerous activity, and private passengers are not trained astronauts. In 2014 a Virgin Galactic test flight broke apart, killing one pilot and injuring another; in 2022 an uncrewed Blue Origin capsule failed in flight, though no one was aboard. Companies ask customers to sign informed-consent documents, and in the United States regulators have been limited by a rule that prevents the Federal Aviation Administration from imposing detailed safety requirements on human spaceflight participants. That rule was intended to protect a young industry, but it is questionable whether it should continue indefinitely.

The word 'democratisation' is also used too freely. A suborbital ticket has been priced at hundreds of thousands of dollars, and an orbital seat at tens of millions. That is not access for ordinary people; it is a market for the very rich. There is nothing inherently wrong with wealthy people buying extraordinary experiences, but it should not be confused with exploration or with public science. Public money is scarce, and the case for spending it on private tourism is weak.

None of this is an argument for banning private space tourism. The industry has demonstrated genuine technical achievement, and a well-regulated market may produce benefits that no one can yet foresee. But we should not accept the industry's own account of itself. Regulation should come before expansion, not after it. Companies should pay for the emissions their rockets produce. They should publish detailed safety data, and accept that some claims about inspiration remain unproven. Public money should continue to fund scientific missions, not tourist ones.