IELTS Academic Reading Practice Test 190

A metal detector is a deceptively simple instrument. It carries a coil of wire through which a current flows, and it reports a change in the electromagnetic field around that coil. Nothing about the principle requires the object to be visible, so the device can find a coin beneath a layer of soil or a weapon beneath a coat. That is why one basic design serves soldiers clearing a minefield, security officers at an airport and amateur historians in a ploughed field.

The earliest attempt to use such a device on a person of consequence came in 1881. President James Garfield had been shot in July of that year, and the bullet had not been found; the surgeons probing the wound were working almost blind. Alexander Graham Bell, already famous for the telephone, assembled an instrument he called an induction balance. It consisted of two coils arranged so that their electrical effects cancelled one another out; when metal came near, the balance was disturbed and a faint sound in a telephone earpiece revealed it.

Bell tested the apparatus on volunteers, and it responded reliably to metal placed near the skin. The attempt on the President failed for a reason that has troubled detectorists ever since: the surroundings were full of metal. Garfield lay on a bed with a metal frame and metal springs, and the instrument reacted to these as readily as to the bullet. The coils also had to be held at an exact distance from the body, which was almost impossible in a sickroom. Garfield died in September 1881.

For the next forty years the device remained a curiosity. Its second life came from an unexpected direction. Gerhard Fisher, a German-born engineer who had settled in California, was building radio direction-finding equipment when he noticed that his apparatus responded to samples of mineral ore. In the 1920s and 1930s he built portable detectors for prospectors, and his instruments were sold for that purpose.

The Second World War turned the metal detector into an instrument of survival. Clearing mines from a beach or a road was slow, murderous work, and soldiers needed to find buried casings without touching them. A Polish signals officer, Józef Kosacki, produced a design in Britain in 1941 that was adopted by the British army: two large circular coils on a long pole, wired to a battery and earphones. Because the coils were swept well ahead of the searcher, the operator could remain upright and cover ground quickly.

Modern detectors work on one of two principles. The first, very low frequency, uses a transmitter coil carrying an alternating current and a receiver coil that listens for the reply. The field from the transmitter induces currents in a metal object, and these produce a field of their own. The strength of the reply, and its timing, reveals something about the target's size and likely identity, because iron and metals such as gold return the signal differently. The second principle, pulse induction, sends brief bursts of current and measures how quickly the resulting eddy currents decay; such machines cope better with wet sand and mineralised soil but are poorer at distinguishing one metal from another.

Airports adopted the technology in the early 1970s, when a wave of hijackings made the screening of passengers unavoidable. The walk-through gateway, a pair of upright panels containing coils, was designed not to find a buried coin but to detect the mass of a knife or a gun on a person passing between them. Its settings must settle a permanent argument: a machine set too low misses weapons, while one set too high stops travellers over the buckles of their belts and the studs in their shoes. X-ray examination of hand luggage came later, and the two technologies have been paired ever since.

Cheap transistors changed the market in the 1960s and 1970s. A detector that had once been a professional instrument became a piece of weekend equipment, and a hobby grew up around it, with clubs, magazines and rallies. Most of what these machines find is rubbish: bottle tops, ring pulls and scraps of agricultural iron.

Occasionally the rubbish is interrupted by something extraordinary. In July 2009 a hobby detectorist, Terry Herbert, was searching a field in Staffordshire when his machine signalled a large mass of metal. The excavation that followed recovered more than 3,500 objects of gold and garnet, almost all of them fittings from weapons and armour made in the seventh or eighth century. The Staffordshire Hoard is the largest collection of Anglo-Saxon gold ever found, and it was discovered by exactly the kind of unsupervised searching that many archaeologists distrust.

The objection is not snobbery. A find lifted from the soil loses the information that surrounds it: the depth at which it lay, the ditch it was buried in, the pottery beside it. Archaeology is largely a science of context, and a hoard without context is a collection of beautiful objects with much of its meaning removed. England and Wales have tried to reconcile the two interests through a voluntary scheme for reporting finds and a law obliging finders to declare hoards of precious metal.

The metal detector, then, has changed remarkably little in principle since Bell's induction balance, and it remains a machine that reports the presence of metal and nothing more. What has changed is the care with which its signals are read, and the rules that decide what happens to what it finds.

An optical telescope's sharpness is limited by the width of its main mirror: the wider the mirror, the finer the detail that can be resolved. Radio telescopes obey the same law of physics, but with a cruel twist. The radiation they collect has a wavelength thousands of times longer than that of visible light, and the sharpness of any telescope depends on the ratio between the wavelength it receives and the width of its aperture. A single radio dish would therefore have to be kilometres across to match the performance of a modest optical instrument. Since such a dish cannot be built, radio astronomers take a different approach: instead of one enormous eye, they use many smaller ones separated by large distances, and combine what they receive.

The technique is called interferometry, and its logic is elegant. Two dishes a certain distance apart receive the same wavefront at slightly different moments. When their signals are brought together and compared, the pattern of agreement and disagreement between them carries information about the structure of the source. In effect, the pair behaves like a single telescope whose diameter equals the distance between the dishes, and this distance, known as the baseline, can be far greater than any physical dish. Adding antennas increases the total collecting area, and therefore the sensitivity, without any need to build a larger dish.

The Very Large Array in New Mexico, inaugurated in 1980, uses 27 dishes, each 25 metres across, mounted on railway tracks so that they can be rearranged into different configurations. In northern Chile, the Atacama Large Millimeter/submillimeter Array, known as ALMA, stands on a plateau about 5,000 metres above sea level and observes at millimetre and submillimetre wavelengths. The site was chosen because it is exceptionally dry: water vapour absorbs radiation at precisely those wavelengths, and the plateau is among the driest places on Earth. ALMA's 66 antennas are repositioned by two custom-built transporters, so that the array can be spread out for fine detail or drawn together for a wider view.

Baselines can be extended much further by linking telescopes that do not share a site at all. In very long baseline interferometry, dishes on different continents observe the same object at the same moment, each station recording the incoming signal and timing it against a local atomic clock. The recordings are then brought together and matched, so that an array spanning thousands of kilometres behaves as a single instrument of that size. The technique yields the sharpest images in astronomy, but it demands extraordinary care: a timing error of a few billionths of a second would ruin the result.

Combining the recordings is a formidable computing task. The signals are digitised and passed to a specialised supercomputer called a correlator, which compares every pair of inputs and averages the results. The volume of data is enormous. When the Event Horizon Telescope observed the black hole at the centre of the galaxy M87, the recordings from each station were far too bulky to send across the internet, so they were written to hard disks and flown to the correlation centres.

The scientific returns have been considerable. Radio arrays have traced the jets of matter flung out by active galaxies, revealed how stars form within clouds of gas and dust, and mapped the structure of our own galaxy behind the dust that hides it from optical telescopes. In 2019 the Event Horizon Telescope published the first image of the shadow of a black hole, and three years later it produced a comparable image of the much smaller black hole at the centre of the Milky Way. In 2023 several collaborations reported evidence of a background of gravitational waves, inferred from minute variations in the arrival times of pulses from rapidly spinning neutron stars.

Interferometry nonetheless has limitations. The most serious is interference from human technology: mobile phones, microwave ovens, aircraft radar and satellites all emit at frequencies that astronomers want to observe, and a weak cosmic signal can be drowned out by a distant transmitter. Some observatories are therefore surrounded by legally protected quiet zones, and array designers try to place antennas in remote valleys screened by hills. Calibration is a second difficulty. The atmosphere bends and delays radio waves, and each antenna's electronics drift with temperature, so the raw data must be corrected before it can be combined.

The next generation of arrays is already under construction. The Square Kilometre Array will consist of thousands of antennas in South Africa and in Western Australia, with a collecting area approaching a square kilometre and its headquarters at Jodrell Bank in England. Its designers expect it to survey the sky far faster than existing instruments and to test ideas about the earliest stars. Low-frequency arrays such as LOFAR, which uses thousands of simple wire antennas spread across Europe, are meanwhile opening a part of the spectrum that has barely been explored.

All of this depends on software quite as much as on steel. An image from an array is not taken but reconstructed, by algorithms that fill in the gaps left by the finite number of baselines. As arrays grow larger and more widely spread, the computing burden grows faster than the hardware, and the ability to process data has become as important a constraint as the sensitivity of the dishes themselves.

Few public services are launched with as much confidence as a free Wi-Fi network. A city announces that its main square, its library and its railway station will be covered; there is a photograph of officials holding a phone; and for a year or two the service works. Then the hardware ages, the contract expires, and the network either disappears or continues unnoticed and unmaintained. My argument in this article is not that these schemes are pointless. It is that they are worth doing, for specific purposes and in specific places, and that the claims made for them are usually far larger than the service can bear.

The case for public access begins with a change in how ordinary life is conducted. Applying for a job, claiming a benefit, booking a medical appointment, paying a bill and completing schoolwork now normally require an internet connection. A household without one is not simply inconvenienced; it is shut out of processes that were designed on the assumption that everyone is online. Public libraries have provided free computers and advice for decades, and their terminals are heavily used by jobseekers, students and people who have recently arrived in the country. A hotspot in a library or a community centre extends that service to people who own a phone or a laptop but cannot afford a monthly connection.

Cost is the second argument, and it is a real one. Wi-Fi uses a part of the radio spectrum that is free to use, unlike the frequencies licensed to mobile operators, and the equipment needed to cover a square or a concourse is modest. Compared with almost any other piece of public infrastructure, a network of access points is cheap to install.

Cheap to install, however, is not cheap to run. The bills that matter arrive later: for bandwidth, for electricity, for repairing units that are vandalised or corroded, and for replacing equipment every few years as standards change. This is where many municipal schemes have foundered. A network built with a grant has no budget for its own maintenance, and the political credit for opening it belongs to one administration while the cost of keeping it alive falls to the next. The result is a service that works well in the year of its launch and erratically thereafter.

Quality is the third difficulty, and it is the one least often admitted. A single access point shares its capacity among everyone connected to it, and its signal weakens with distance and with every wall in between. A public network is therefore not a substitute for a connection in the home, and to describe it as one is to mislead precisely the households that most need help. A child revising for examinations, or an adult attending an interview by video, needs a reliable connection in a private room, not a bench in a square where the signal comes and goes.

Privacy deserves more attention than it usually receives. Many public networks ask users to identify themselves with a telephone number or an email address before they can log on, and the operator can see which devices connect and when. Kiosks that combine a screen, a camera and an access point gather information about everyone who walks past, not only about those who log on, and there have been sharp disagreements about how long such records are kept and who may inspect them. For most users this hardly matters. For a person searching for a support service, or contacting a lawyer, the knowledge that the session is being logged is a reason to stay away.

There is also an uncomfortable point about who actually benefits. A hotspot is most useful to someone who already owns a smartphone or a laptop; someone who owns neither gains nothing whatever from it. Free Wi-Fi cannot help a person who lacks a device, a reliable place to sit or the confidence to use the equipment. Devices, training and human help are as important as bandwidth, and they are usually the parts of a digital inclusion programme that are cut first.

None of this means that public money should go to hotspots alone. Community-owned networks, in which residents and local organisations build and run their own infrastructure, and municipal fibre schemes, in which a city lays cables and sells access wholesale, have been tried with some success in various countries. Subsidised home connections for low-income households reach the excluded more directly. The sensible question is not whether a city should offer free Wi-Fi, but how much benefit each unit of spending achieves compared with the alternatives.

My conclusion is therefore unfashionably moderate. Public Wi-Fi should be kept and properly funded in libraries, community centres and transport interchanges, where it serves travellers, students and people whose home connection has failed. It should be designed so that privacy is protected by default, and the data it generates should be made public so that residents can judge whether it is worth the money. But it should be understood as a complement to home broadband, not as a substitute for it, and no politician should be allowed to present a hotspot as a solution to poverty. A modest service, honestly described, is worth defending. A grand claim, quietly abandoned, helps nobody.