IELTS Academic Reading Practice Test 128

In October 1982 a small silver disc went on sale in Japan, and within a decade it had displaced the long-playing record in much of the world. The compact disc stored music as a stream of numbers rather than as the wobbling groove of a vinyl record, and it was read by a beam of light that never touched the surface. Buyers were promised that the disc would last forever and that its sound was perfect. Neither claim proved quite true, but the technology nevertheless transformed the way recorded music was made, sold and listened to.

The idea of storing sound on a disc read by light was older than the compact disc itself. In 1966 an American physicist named James Russell, working at the Battelle Memorial Institute in the state of Washington, filed a patent for an optical system that recorded digital signals as microscopic pits on a rotating disc and read them back with a laser. His design was decades ahead of the hardware needed to build it: lasers were expensive and unreliable, and the electronics required to process digital audio in real time did not yet exist. Russell's patents were eventually licensed, but no consumer product came of them.

Meanwhile two companies were approaching the problem from different directions. Philips had developed the LaserDisc, a video disc that stored pictures as analogue signals and was read by a laser, while Sony had built a reputation for digital audio and had already worked on pulse-code modulation, a method of turning sound into numbers. In 1979 the two firms, which had been rivals, agreed to cooperate on a single digital audio disc. The arrangement was unusual. Philips contributed the optical system and the physical design of the disc, while Sony contributed the digital encoding and the error-correction methods that would make a scratched surface playable.

The details were settled by a joint task force of engineers who met repeatedly in 1979 and 1980. Their hardest problem was error correction. A disc is a hostile environment for data, because dust, fingerprints and tiny manufacturing flaws can destroy the signal read from thousands of consecutive pits. Sony's solution, known as Cross-Interleaved Reed-Solomon Coding, spread the data from each moment of music across a wide area of the disc, so that a scratch would damage a small part of many samples rather than all of a few, and the missing information could then be reconstructed mathematically.

The disc's diameter of 120 millimetres and its capacity of 74 minutes are often attributed to a wish to fit a single performance of Beethoven's Ninth Symphony, said to have been a favourite of the Sony executive Norio Ohga, who had trained as an opera singer. The story has been repeated so often that it has become part of the format's folklore, but the engineers involved gave several explanations over the years, and the real reason may simply have been a compromise between the two companies' prototypes.

The system that emerged is easily described. The music is measured 44,100 times every second, and every measurement is stored as a 16-bit value, which gives a wider range between the quietest and loudest sounds than any analogue tape could manage. The numbers are stored along a single spiral track that begins near the centre of the disc and winds outwards, and the track consists of microscopic pits separated by flat areas known as lands.

A laser beneath the disc shines through the transparent plastic, is reflected by a thin aluminium layer and is detected by a photodiode; changes in the reflected light correspond to the pits, and therefore to the numbers. Because the beam is focused through the plastic, dust on the outer surface is out of focus and does no harm.

In June 1980 the two companies published the technical specification that became known as the Red Book, and the first players and discs reached shops in Japan in October 1982. The first commercially released disc is generally identified as Billy Joel's album 52nd Street, and the first player was Sony's CDP-101, a machine that cost far more than a conventional record deck. Europe and the United States followed in 1983, and by the end of the decade record companies were closing their vinyl pressing plants.

Other uses soon appeared. In 1983 the companies published a further specification, the Yellow Book, which defined CD-ROM, a disc on which any digital data — text, images or software — could be stored, and the first drives reached the market in the mid-1980s. A single disc held around 650 megabytes, at a time when the hard disk of a personal computer was measured in tens. Reference works, encyclopedias and games were sold on disc, and the CD became the main carrier of software before the internet took over that role.

Recordable and rewritable discs extended the format's life, allowing anyone with a computer to make a personal compilation of favourite tracks. Yet the CD's dominance was shorter than it looked. Compression formats such as MP3 made it possible to carry hundreds of albums in a pocket, and by the 2010s streaming had made the disc an optional extra. Sales collapsed and the factories that had pressed millions of discs a year fell silent. The compact disc has not vanished — it survives in archives, in cars and among collectors — but its time as the default way of owning music lasted barely a quarter of a century.

In 1911 the Dutch physicist Heike Kamerlingh Onnes, working in Leiden, was studying how metals behave at extremely low temperatures. He had recently succeeded in liquefying helium, which boils at about 4.2 kelvin above absolute zero, and this gave him access to a coldness that no one had reached before. When he cooled a thread of mercury below that point, its electrical resistance did not merely fall as expected: it dropped abruptly to zero. Later experiments showed that a current could circulate in a superconducting loop for long periods with no applied voltage at all. Onnes called the phenomenon superconductivity, and for the next half-century nobody could explain it.

The explanation arrived in 1957, in a theory devised by John Bardeen, Leon Cooper and John Schrieffer and usually known by their initials as BCS theory. In an ordinary metal, electrons are impeded by collisions with the vibrating atoms of the crystal lattice. In a superconductor, however, a subtle interaction between the electrons and those vibrations causes them to pair up. These Cooper pairs behave as a single quantum entity, and the whole population of pairs moves in step, so that the individual collisions which would scatter a normal electron cannot occur. The result is a current that flows without resistance, and the work earned the three physicists a Nobel Prize in 1972.

Superconductors also do something that a merely excellent conductor cannot. In 1933 Walther Meissner and Robert Ochsenfeld showed that a superconductor expels a magnetic field from its interior instead of allowing the field to pass through it. A hypothetical perfect conductor would do the opposite, trapping within itself whatever field was present when it was cooled. This expulsion, known as the Meissner effect, allows a small magnet to float above a superconducting pellet, and it is the basis of proposals for frictionless bearings and for trains that hover above their tracks.

Almost all superconductors used in engineering are alloys of niobium, particularly niobium-titanium, which becomes superconducting below about nine kelvin and can be drawn into long, flexible wires. Such a wire carries a current only while the current itself and the magnetic field around it remain below certain limits; beyond them the material reverts to ordinary behaviour and the coil heats up. Engineers therefore speak of three critical values — temperature, magnetic field and current density — and a practical device must stay inside all three. Niobium-tin, which tolerates higher temperatures and stronger fields, is used where niobium-titanium is not sufficient.

The largest use of superconductors today is in medicine. An MRI scanner builds its images with a powerful magnet, and only a superconducting coil can maintain a field of the required strength in a hospital room without an enormous power supply. Particle accelerators depend on the same technology: the Large Hadron Collider, a ring twenty-seven kilometres in circumference, steers its beams with more than a thousand superconducting magnets. Superconducting coils are also being tested in motors and generators, in cables intended to carry electricity through crowded cities, and in experimental fusion reactors, where they confine a plasma hotter than the centre of the sun.

The obstacle is temperature. Niobium alloys must be cooled to within a few degrees of absolute zero, which in practice means liquid helium, an expensive substance produced in only a handful of countries. The refrigeration plant of an MRI scanner is a substantial part of its cost, and a large accelerator consumes a great deal of energy simply keeping its magnets cold. Superconducting magnets can also quench: if a small section warms and loses its superconductivity, the current is suddenly forced through ordinary metal, generating heat, and the energy stored in the coil must be dumped safely within seconds.

In 1986 Georg Bednorz and Alex Müller, working at IBM's research laboratory near Zurich, found superconductivity in a ceramic material based on copper oxide, at a temperature that surprised almost everyone. Within a year a related compound containing yttrium, barium and copper had been shown to superconduct at 92 kelvin, which is above the boiling point of liquid nitrogen at 77 kelvin. Since nitrogen is cheap and is liquefied on an industrial scale, this was a genuine breakthrough. But the new materials were brittle ceramics, and turning them into long wires able to carry a large current proved far harder than it had been for niobium alloys. The two discoverers shared the Nobel Prize in 1987.

Research since then has produced many more superconducting compounds, including magnesium diboride in 2001 and a family based on iron, and it has pushed the record temperature steadily upwards. The most striking results have come from hydrogen-rich materials squeezed between the jaws of a diamond press at pressures more than a million times that of the atmosphere, where superconductivity has been reported at temperatures approaching 250 kelvin. Such pressures are far beyond anything achievable inside a machine, and the materials themselves are unstable, so for the moment these are laboratory curiosities. The goal of a superconductor that works at room temperature and ordinary pressure remains unmet.

The practical outlook is therefore mixed. Superconductivity will not soon replace copper in domestic wiring, and the dream of a national grid that loses nothing in transmission depends on materials that do not yet exist. In the markets where a very strong magnetic field is worth the cost of refrigeration, however, the technology is already unbeatable, and the slow improvement of high-temperature wires suggests that those markets will grow. Superconductivity is less a single invention than a family of materials, each with its own difficult balance between temperature, field strength and the price of keeping it cold.

Hardly a week passes without a museum announcing that another part of its collection has been put online. Virtual tours, three-dimensional scans of sculpture and high-resolution photographs of paintings have become standard features of institutional websites, and the language that accompanies them is almost always the same: the collection, we are told, is being opened up, made accessible, democratised. The sums invested are considerable, even if they are rarely disclosed, and they have bought a great deal of equipment, cataloguing and platform development. It would be churlish to deny that something valuable has been achieved. But the claim that a museum's holdings are now available to everyone deserves closer examination.

The clearest benefits are scholarly. A historian studying a medieval manuscript no longer needs to travel to the library that holds it and wait for an appointment in a supervised reading room. A good digital surrogate can be magnified, compared side by side with another copy, and examined under different wavelengths of light, all without handling the original at all. Multispectral imaging, which captures information invisible to the naked eye, has recovered writing that was erased from parchment when the leaves were scraped clean and reused centuries later, and infrared reflectography has exposed underdrawing beneath the paint of panel paintings. For objects too fragile to be displayed, or held in storage for most of the year, digitisation is not a poor substitute for the real thing; it is the only form of access there is.

For the general public the picture is less straightforward. The digital audience is enormous, but it is also shallow. Most people who look at a museum's website arrive from a search engine, spend a few minutes on a single page and never return. Some commentators have predicted that virtual tours will empty the galleries, yet there is little evidence that this has happened. What the website does is to make a first contact possible; what it cannot do is supply the encounter that follows.

Digitisation is also far more expensive than it looks. The camera work is the least of it. A high-quality three-dimensional scan of a single complex object can take hours of technician time, and every image must then be described, dated, credited and checked for copyright before it can be published. Cataloguing of this kind frequently costs more than the photography it accompanies, and it is the part that funders are least eager to pay for. Storage is not a one-off purchase either: files must be copied, checked and migrated to new formats as software changes, a commitment that continues for as long as the museum intends to keep its digital collection alive.

That commitment is easily underestimated. Digital preservation specialists insist that it is a process rather than a product, and they have watched institutions publish ambitious online collections that quietly decayed because nobody had budgeted for their maintenance. A file that can no longer be opened is worse than no file at all, since it creates an illusion of preservation. The same money spent on a climate-controlled store would at least have kept the objects themselves stable.

There is a further problem, which receives less attention than it should. Digitisation is inevitably selective. Museums hold millions of objects, most of them in storage and many of them undocumented; the ones that are scanned tend to be those that are already famous, already photographed and already in demand. The result can be a digital collection that flatters the institution's highlights while leaving the obscure majority invisible. Copyright adds another constraint, and so do the cultural protocols of source communities: some objects were never meant to be seen by outsiders, and a photograph placed on an open website can be an act of appropriation rather than of generosity.

Critics of digitisation often argue that a reproduction diminishes the original, that the purpose of a museum is a physical encounter with a thing made by human hands, and that a screen flattens what a gallery amplifies. There is something in this, but the argument is weaker than it seems, since museums have always relied on reproductions: engravings, plaster casts and colour plates were the digital images of their day. What is new is not mediation but scale, and the risk that comes with it. If a director can point to a well-attended online gallery, the case for spending money on the building itself becomes harder to make.

My own view is that digitisation should be welcomed, funded and, above all, understood for what it is: a form of collections care, not a replacement for the collection. Its strongest justification is access to fragile material and the research that such access makes possible. Its weakest is the assumption that a picture on a website is equivalent to a visit. The test of a digital programme is not how many images it has produced but whether it has changed what a scholar, a student or a curious member of the public can actually do. By that measure the results so far are encouraging, but they are not the revolution that press releases describe.