IELTS Academic Reading Practice Test 185

In the late 1670s a French-born physicist named Denis Papin showed the Royal Society in London a heavy metal vessel with a lid that could be clamped shut. Inside it, bones and other tough materials were boiled until they became soft enough to crush. Papin called the device a 'steam digester', and it was the direct ancestor of the modern pressure cooker. It worked because of a principle that had previously been understood only vaguely: the temperature at which water boils rises as the pressure above it increases. Papin's vessel could therefore reach temperatures well above the boiling point of ordinary water, and it cooked food in a fraction of the usual time.

Papin was not a cook but a physicist who had worked as an assistant to Robert Boyle, and he understood that air has weight. At sea level the atmosphere presses on the surface of a pan and raises the boiling point of water to 100°C; high on a mountain, where there is less air above, water boils at a lower temperature and food takes longer to soften. Papin's insight was that the same effect could be produced in the opposite direction. If steam were prevented from escaping, the pressure inside the vessel would rise above atmospheric pressure, and so would the temperature of the water within it.

A sealed pot, however, is a bomb. Papin's solution was a valve held shut by a weight, which lifted automatically once the pressure inside exceeded a chosen level and allowed steam to escape. The idea survives in the weighted regulator found on many stove-top cookers today, which rocks and hisses when the correct pressure has been reached. Even so, early digesters were dangerous objects, and Papin himself acknowledged the risk of explosion. The pressure that made cooking fast also made failure violent.

Papin had ambitious ideas about what his machine could do. He suggested that it might extract nourishment from bones and other cheap materials, and thus help to feed the poor, and he demonstrated dishes prepared in it to fellow members of the Royal Society. For more than two centuries, though, the digester remained a piece of laboratory apparatus and a specialist tool used by confectioners and glue-makers; it did not enter ordinary kitchens. What changed that was a different application altogether, in which the aim was not to cook food but to kill micro-organisms.

In 1879 Charles Chamberland, a collaborator of Louis Pasteur in Paris, built a sealed chamber that admitted steam under pressure and used it to sterilise medical instruments and culture media. His device, the autoclave, worked on exactly the same principle as Papin's digester, and it became indispensable in hospitals and laboratories. Autoclaves are still used, and their standard conditions - roughly 121°C, with steam about one atmosphere above normal pressure, held for fifteen minutes - show plainly what raised pressure does to temperature.

Domestic pressure cookers appeared in the early twentieth century, first as heavy vessels intended mainly for preserving vegetables at home, then as lighter aluminium pans aimed at ordinary households. The design that became familiar was simple: a thick pan with a locking lid, a rubber gasket to seal it, and a regulator on top. Advertisements promised enormous savings of fuel and time. At the 1939 New York World's Fair, crowds watched demonstrations of a cooker with a lid that could be sealed in seconds, and demand grew rapidly. By the late 1940s the appliance was a common wedding present in the United States.

Prosperity brought problems. Reports of lids flying off and scalding accidents multiplied, and safety became a public concern. Manufacturers responded with interlocking lids that could not be opened while pressure remained, and with a second, emergency plug that would release if the main vent became blocked. The early cookers were also heavy and needed constant attention. With the arrival of cheap frozen food and the microwave oven in the 1970s, sales fell sharply, and the pressure cooker came to be seen as old-fashioned.

The revival came from electronics. In 2010 a Canadian engineer, Robert Wang, launched an electric multi-cooker controlled by a microprocessor, which combined a pressure cooker with a slow cooker and other functions in one unit. Because the machine senses temperature and pressure and controls the heat itself, it removed most of the attention that stove-top cooking demanded, and its popularity renewed interest in pressure cooking generally. Similar appliances are now made by many manufacturers.

The physics has not changed since Papin's day. Water in a sealed pot at about one atmosphere of extra pressure boils at around 121°C, which is enough to break down the tough connective tissue in meat and to dissolve the pectin that holds vegetables together in a third of the time that simmering would require. Because the food cooks quickly in a small amount of liquid, cooks claim that fewer water-soluble vitamins are lost than in boiling. What has changed is the engineering around the physics: the reliability of seals, the precision of valves and, most recently, the electronic control of heat.

For three hundred years, then, the pressure cooker has illustrated a familiar pattern in the history of technology. The underlying principle was discovered long before the product was safe or convenient enough to be widely used, and it took advances in unrelated fields - metallurgy, rubber, electronics - to turn a laboratory curiosity into an everyday object.

Anyone who has flown in a jet airliner knows the sound: a low, continuous roar, strong enough to make conversation difficult and tiring over several hours. Ordinary earplugs, or thick cushions over the ears, reduce such noise by blocking it - the approach called passive isolation - but they weaken high frequencies far better than low ones, and low-frequency sound is exactly what engines produce in abundance. Active noise cancellation takes a different approach. Instead of blocking sound, it generates a second sound designed to cancel the first, using small microphones and loudspeakers built into the ear cups.

Sound travels as a wave of alternating high and low pressure. Where two waves meet, their pressures add together: if the compression of one coincides with the compression of another, the result is louder, but if a compression meets a rarefaction - a region where the pressure is below average - the two partly annihilate each other. This is destructive interference. The electronics in a headset sample the incoming noise with a microphone, invert the signal so that its peaks become troughs, and play the inverted copy through the loudspeaker. For the trick to work, the copy must be accurately timed as well as accurately shaped: if it arrives too late, its peaks no longer line up with the troughs of the original.

The idea is old. A German physicist, Paul Lueg, applied for a patent in the 1930s for a system that would use a microphone, an amplifier and a loudspeaker to silence unwanted sound; the electronics of the period were too crude and bulky to make it work outside a laboratory. In the 1950s engineers at the American company RCA, among them Harry Olson, described a feedback arrangement in which a microphone close to the loudspeaker monitored the result and corrected it. These were demonstrations rather than products, and the practical obstacle remained the same: the cancellation had to be calculated and delivered within a fraction of a millisecond.

The first commercially successful headset came from an unexpected direction. According to the company's own account, the founder of the audio firm Bose, Amar Bose, was on a flight in 1978 when he put on the airline's headphones and found that the engine noise drowned out the music. He began a research programme that lasted more than a decade, and in 1989 the company sold its first noise-cancelling headset, intended for pilots. A consumer version followed in 2000. The early products were expensive and heavy, but they established the category, and by the 2010s the technology had spread to earbuds costing a small fraction of the price.

Modern headsets use two arrangements, often together. In a feedforward system a microphone on the outside of the cup samples the noise before it reaches the ear; the processor then predicts what the sound will be like at the eardrum and generates the opposite. In a feedback system the microphone sits inside the cup, close to the ear, and measures what remains after cancellation, adjusting the output continuously. Each has weaknesses. Feedforward designs can be misled by the shape of the wearer's head and by how well the cup fits; feedback designs are stable only within limits, because the corrected sound must not be picked up and amplified again. A hybrid, with microphones in both positions, is the commonest solution.

Cancellation is easiest at low frequencies. A sound of 100 hertz has a wavelength of about three and a half metres, so a small error in timing barely shifts its phase; at 4,000 hertz the wavelength is under nine centimetres, and a delay of a fraction of a millisecond can turn a trough into a peak. This is why headsets silence the drone of engines and air-conditioning but leave speech, alarms and a crying baby largely intact. Manufacturers regard that selectivity as a virtue, since it preserves awareness of the surroundings, but it distinguishes active cancellation from hearing protection, which aims to reduce everything.

Neither approach is perfect. Passive isolation still matters, especially for high frequencies, and it also shapes the low-frequency response of the cup so that the electronics have less work to do. Some users report a sensation of pressure or fullness in the ears when cancellation is switched on, even though the pressure in the ear canal has not changed; the effect appears to be caused by the near-silence itself rather than by any physical force. Others find that the system amplifies wind noise when microphones on the outside are struck by moving air.

Beyond aviation, the technology is used in industrial hearing protection, in call centres and open-plan offices, and in the earbuds that many people wear while travelling. Its spread has prompted questions about safety. Several studies have examined whether pedestrians or cyclists who wear such devices are less likely to hear approaching vehicles; the picture is complicated, because the devices also remove the fatigue that constant noise produces. In practice, manufacturers increasingly offer a transparency mode that lets outside sound through, and safety campaigners have asked whether people should use the devices on the road at all.

Current research aims at wider bandwidth, faster processing and cheaper components. Adaptive algorithms can be trained to follow changes in background noise, and machine learning is beginning to be used to distinguish steady noise from speech, so that a headset can cancel one while passing the other through. If these systems succeed, the roar of an engine may one day vanish altogether, leaving only the conversation.

A few years ago, buying a ticket for almost anything meant queueing at a window, handing over money and receiving a piece of card. Today, in much of the world, the card has been replaced by a pattern of black-and-white squares on a phone screen. Concert halls, railways, football grounds and museums have moved in the same direction, and the language they use is telling: paper is described as waste, and the shift is presented as a step towards a cleaner, faster, fairer service. I use mobile tickets myself and would not willingly go back. Yet I am uneasy about the speed at which the alternative is being withdrawn.

The case for digital tickets is real. Issuing, posting and checking paper costs money, and a barcode read by a scanner is faster at a crowded gate than a torn stub. Tickets cannot be lost or counterfeited as easily, and a seat can be reallocated at once if a booking is cancelled. Venues also learn who came, when they arrived and what they bought, information that is genuinely useful for planning. For a large organisation the savings in printing and postage alone can be considerable. On any narrow accounting of cost and speed, the digital ticket wins.

The environmental argument, however, is less solid than it appears. A thermal paper ticket is small, but it is only one element in a chain that includes servers, networks, card readers and the phones themselves, which are energy-intensive to produce and are replaced every few years. A life-cycle comparison is complicated, and I have seen no convincing study showing that the total burden is lower. 'Paperless' is an appealing word, but it describes paperwork, not impact.

The more serious worry is access. A ticket held on a phone requires a charged battery, a reliable signal, a working app, an account, a means of payment and the confidence to use all of them under time pressure. Each requirement excludes somebody: the pensioner without a smartphone, the teenager whose phone has run out, the visitor whose foreign card is refused by the app, the family that booked together but must now hold four tickets on one device. Venues insist that staff will always help, but the queue at the box office that used to serve everyone now serves only those who cannot cope with anything else.

Systems also fail. Networks go down, payment processors refuse transactions, and scanners stop reading. A paper ticket continues to work when the power does, which is a modest but genuine advantage at the gate of a stadium. The organisations that handle the transition best are those that assume failure and plan for it, with printed lists of names, codes downloaded in advance and staff who can admit a customer without a device.

Then there is data. A paper ticket reveals nothing when it is handed over. A digital one can be linked to a name, a bank card and a telephone, and can record not only that somebody attended but when they arrived and where they went afterwards. Much of this is harmless, and some of it is useful to the customer. But the infrastructure that collects it is rarely designed with limits on how long it is kept or who can see it, and the terms accepted at the point of purchase are read by almost nobody.

Digital tickets also change what a ticket is. A paper ticket could be bought as a present, given to a friend or sold on. Many digital tickets are tied to an account and cannot be passed on at all, which suits organisers who are fighting touts but also removes a small freedom that customers used to take for granted. The loss is not dramatic, but it is typical of the way digital systems quietly redefine the thing they replace, and customers rarely notice until they want the freedom back.

Not every objection deserves weight. Collectors mourn the printed stub, with its design and its punctured edge, and there is a mild pleasure in keeping a ticket from a memorable evening. That is a genuine loss, but it is sentimental rather than serious, and sentiment cannot justify keeping an expensive parallel system alive on its own.

What, then, should be done? I do not accept the argument that paper must be preserved indefinitely because some people find phones difficult; that is a reason to design better systems, not to refuse progress. Nor do I accept the claim of organisers that help is always at hand, when the help consists of one member of staff and a growing queue. My position is that digital ticketing should be the default only where its infrastructure is dependable and where an equivalent alternative is genuinely available to those who cannot use it, and that this condition is not yet met in most places.

Until it is, the paper ticket should be retained alongside the app, at modest cost. The move to digital ticketing is not a single decision to be taken once; it is a promise about reliability, fairness and privacy that has to be kept, and the sensible thing is to keep the old system until the new one can be trusted with everyone who uses it.