Domestic science - Why Can't Elephants Jump?: And 101 Other Tantalising Science Questions - New Scientist

Why Can't Elephants Jump?: And 101 Other Tantalising Science Questions - New Scientist (2010)

Chapter 3. Domestic science


Rip off

Try tearing a piece of sticky tape across its width and you’re asking for trouble but, if you nick it with a sharp object first, it tears easily. Why?

Daniel Albert

Science Museum, London, UK

When a material has a crack in it, any stress concentrates around the crack tip. The same applies to sticky tape. The sharper the crack the more stress becomes concentrated at the tip. Hence, even under small loads a crack can propagate through the material. Drilling holes around crack tips in metals or plastics stops them propagating because the crack tip blunts and is less liable to spread.

Alex McDowell

South Ruislip, Middlesex, UK

Imagine stress as lines of tension within the stretched material: those lines must detour round the ends of any crack they meet. The deeper and sharper the crack, the more crowded the stress lines passing its sharp edge, so stress concentrates there. In rigid material the concentrated stress favours crack growth. Conversely, yielding materials deform cracks, blunting them, dispersing stress and stopping their growth.

Modern plastic tapes of the type described are made of polymers whose long molecules have very little give to them when they are stretched. Manufacturers stretch the tape, orienting the molecules to lie straight and closely parallel in the long direction of the tape. Unoriented tape is weak, but tends to distort and stretch rather than snap. Oriented tape is strong until it is nicked. At the nick its oriented molecules resist distortion and the crack propagates across it as though through scratched glass, only more slowly.

Jon Richfield

Somerset West, South Africa

This is a simple demonstration of stress concentration. The nick causes a reduction in cohesive surface area, allowing the force of the tearing to act along the geometric discontinuity. You will notice that a piece of tape with a hole created by a hole punch is harder to tear than one with a slight cut along the edge - the round hole distributes the force evenly.

Stress concentrations need to be accounted for by engineers during design. Many have learned the hard way. Nineteen US-built Liberty cargo ships used to supply the UK during the Second World War cracked in half, with the splits originating at the square corners of hatches. The design of its successor, the Victory ship, used circular portholes and rounded hatchways. The de Havilland Comet aircraft also suffered accidents caused by stress concentration. In 1954, one broke apart and crashed en route to Cairo. The cause was cracking around the rivets at the corners of the square windows. Aircraft now have rounded windows.

Adam Long

Industrial design student

University of New South Wales

Canada Bay, Australia


On the blink

When I watch digital TV channels from terrestrial transmitters, I have to endure periodic disruptions during which the audio and images start stuttering. I recently realised that the disturbances occur every time motorbikes - and particularly scooters - pass my house. It doesn’t happen with cars. How do scooters disrupt my TV?

Michael Smith

High Wycombe, Buckinghamshire, UK

Petrol engines use an electrically generated spark to ignite the fuel-air mixture. A modern car uses a solid-state electronic ignition system, connected to the spark plugs using cables which reduce the level of electromagnetic radiation from the engine.

In contrast, many motorcycles and scooters, especially those with two-stroke engines, use magneto ignition systems. A magneto is a simple mechanical device involving a coil and a magnet that can generate a high voltage when it is needed. It is connected to the spark plugs using metal cables rather than the high-resistance cables used in cars, which are not well suited to a magneto system. As a result, magneto systems emit much higher levels of electromagnetic radiation than electronic ignition systems.

The high-frequency component of this radiation can interfere with broadcast signals to such an extent that your receiver’s error-correcting routines cannot cope.

Stewart Haywood

The Colony, Texas, US

This problem is worse when receiving terrestrial digital TV than with analogue transmissions. Analogue technology is far more robust and just displays such interference as white dots on the screen. However, with digital technology there is a very high level of coding in the signal waveform. The result is that ignition interference (and indeed other kinds of interference) can disrupt the decoding process in the receiver, causing the picture to stutter or even freeze completely while the decoder recovers and gets back in sync.

Sometimes replacing the aerial cable with a double or triple-screened variety can cut the amount of ignition pick-up and so reduce glitches.

Richard Harris

Malvern, Worcestershire, UK

In the UK analogue TV transmissions are due to be switched off in 2012. Until then, digital terrestrial signals are being transmitted at reduced power to avoid interference with analogue signals. When the digital signal strength is eventually increased, the improved signal-to-noise ratio detected at the receiver should overcome this problem.

I should add that I cannot receive digital terrestrial signals at all. My neighbour can, and suffers from the same problems as your correspondent. We have identified the individual scooters, motorcycles and cars responsible - they are all rust buckets.

If it makes anyone feel better, I watch free-to-air satellite channels with a 1.2-metre dish and find that one particular motorcycle interferes with that too. This is amazing as the signals are all in the gigahertz rather than the megahertz range, and the interference is coming from outside of the line of sight to the satellite.

Adrian Frame

Norwich, Norfolk, UK


Contrary cotton

Some garments made of 100 per cent cotton will hang-dry on the washing line without creases, while other pure cotton items end up covered with stubborn creases that only a steam iron will shift. Why is this? Is it the quality of the cotton, or perhaps the species of cotton plant it comes from? And while we’re on the subject, some cotton towels never really absorb the moisture from your body, however many times they are washed, while others - initially resistant while new - age into the job perfectly well after visiting the washing machine. What’s going on?

Diana Ball

By email, no address supplied

One of the reasons for a difference in crease retention is that cotton varies in its fineness. Fine cotton tends to form fewer creases than coarse fibre cotton. Also, a tight weave will tend to crease more than a loose weave, and woven fabrics will crease more than knitted fabrics.

Washing is responsible for most creases found in the fabric. When cellulose, an essential component of all plants, including cotton, gets wet, the hydrogen bonds holding it in shape are broken. If the fabric is dried in a creased state, new bonds form that hold these creases in place, where they stay, sometimes even after vigorous ironing.

Many cotton fabrics today receive chemical crease-resistant treatments. The presence, type or quality of this treatment is the likely reason you are experiencing major differences in the crease retention of cotton garments.

Regarding towels, one made with pile yarns consisting of fine fibres will dry you much better than a towel made with yarns consisting of coarse fibres. This is because fine fibres form a greater number of finer capillaries that wick the water away from the body more efficiently.

Years ago, all towels were made with loops of fibre, or terry, on both sides of the towel. Today, many are sold with looped terry on one side and a cut pile of single fibres on the other. The fibres on the side of the towel that has the cut-pile tend to separate, which reduces their water-attracting capillary action and by consequence their ability to dry your skin. So while the cut-pile side of the towel has a rich and velvety appearance, the looped terry side is much better for drying purposes.

J. Robert Wagner

Plymouth Meeting, Pennsylvania, US

A very high percentage of what is labelled 100 per cent cotton has been chemically treated to enhance the properties of the garment or linen. Much of this treatment is focused on modifying the surface of the fibre.

In the 1950s cotton fabric was treated with epichlorohydrin, which very effectively prevented wrinkling by keeping the individual fibres straight and a little springy. Unfortunately the compound weakened the fabric, so it was replaced by new treatments, including resin coatings. Many of these coatings are still in use today.

If you have ever visited a fabric shop, you may recall a pungent odour. This comes from the formaldehyde that is used to treat cotton fabrics and is present in a type of phenolic formaldehyde resin. The latest fabric surface treatments include Teflon resins to provide wrinkle resistance and prevent staining.

Some towels and bed sheets are treated with heavier coatings of polymers to give a very soft feel which, while pleasing, has the unintended consequence of repelling water. I personally discard any towels with this permanent softener treatment because they remind me of trying to dry myself with a plastic bag.

Incidentally, it would be prudent to thoroughly wash any new garment before wearing it and allowing it into contact with the skin. Many of the cotton treatments described above can be hazardous for those working with them during manufacture - for example, some fabric softeners can trigger allergic reactions.

Charles Caban

Smyrna, Georgia, US


In the clink

My father-in-law used to tape family mealtime conversations. When played back, the background noise - like silverware hitting plates and doors closing - is surprisingly prominent. Why is it that we filter these sounds out as they happen, but seem unable to filter them out when we listen to the recording?

Gary Yane

Oldenburg, Indiana, US

Microphones are wonderfully objective devices. They detect variations in absolute pressure or the pressure gradient on a particular axis and faithfully transduce these into electrical signals. In contrast our ears have a brain attached, and between them they do a much more subjective job - interpreting our acoustic environment, not just recording it.

Our ears themselves are simple pressure transducers, but we also have ways of working out where sounds are coming from. For this we make use of the relative levels, phase and arrival times of sounds. In addition, the shape of our head distorts the local audio field in a way that we are personally familiar with, and this aids location of sound sources, particularly when we can move our heads.

We detect not only direct sounds but also reverberant ones. The space we are in significantly colours and adds to sounds, mostly in the form of delayed noise from random reflections. This would severely reduce the intelligibility of sound if the brain were not adept at adjusting to these conditions. It works out when the noises arrive, and where from, and can largely ignore them if it so chooses.

When the sound objectively recorded by a microphone is replayed through a single loudspeaker or even a stereophonic system, the random cutlery-clanging reverberant sound that should be all around us is now directly in front of us. Directional and timing cues that the brain would normally use for filtering are now inconsistent or just plain wrong.

Chris Woolf

Liskeard, Cornwall, UK


Monkey business

My son has a game in which you hang small plastic monkeys and gorillas from a plastic network of tree branches. The branch network is attached to an overhanging trunk by a magnet. As more monkeys are hooked on, the network becomes more unsteady until the magnet can hold it up no more. The player who breaks the bond is declared the loser. The game depends on a magnet of a clearly defined strength. But how is this strength determined so accurately during manufacture that it can hold almost all the monkeys (but not quite)?

Pavel Sypchenko

Kiev, Ukraine

The strength of a magnet depends on its material, shape and preparation. When magnets are made these properties are fairly well standardised to within a few per cent of the strength required. For the purposes of this game, that is precise enough. You only need the game to last beyond the first few monkeys and end before you run out of monkeys.

Secondly, there is far greater variation in how the game is played than in how magnets are made. Magnets are exquisitely sensitive to how well they fit together. When polished to fit very closely they can cling to each other amazingly strongly, whereas the merest roughness, dent or sprinkling of dust can weaken their attraction by a large factor. Details of their orientation also affect their strength.

Players of the game are unlikely to pay much attention to such finicky details, so the strength of any set-up is likely to vary more in other ways than in the innate strength of the magnet. Then again, a really thoughtful player, with steady hands and a good eye for which plastic monkey to select and where to hang it, might survive several rounds longer than a slapdash player.

Karl Kracken

Hull, East Yorkshire, UK

Manufacturers are more likely to make a magnet and then test the number of monkeys it can hold before the network of branches will fall, than manufacture the magnet to a specific strength. This way they can supply a number of monkeys in the game box in excess of the magnet’s strength. Supplying a few extra monkeys is a lot cheaper than going through weeks of R&D to create the perfect magnet.

Sohil Patel

London, UK


Rank and dank

Why do wet things smell more than dry ones?

Ben Scullion (age 4½)

Darlington, Durham, UK

Making something wet does not automatically make it more smelly. For instance, a wet clean towel smells no worse than a dry clean towel. However, the presence of moisture does allow the growth of bacteria, assuming that there is organic matter present for the bacteria to eat. As they grow and multiply, bacteria produce a whole range of smelly compounds of the kind you can detect in bad breath, for example. So, given moisture and enough time for bacterial growth, wet things can smell worse. But if you prevented bacterial growth by, say, sterilising the wet item to kill all bacteria, then it wouldn’t develop such a smell.

Simon Iveson

Department of Chemical Engineering

University of Newcastle

New South Wales, Australia

Most chemical components of dank smells are products of microbial activity, and microbial activity requires water. Once the chemicals are present they can reach the nose only by escaping into the air.

Most are fatty acids, amino compounds and the like, with charged chemical groups that readily bind to non-volatile molecules such as large proteins and carbohydrates. Once they have latched onto, say, dry cloth or leather, they cannot float freely into the air so there is not much to smell. However, these charged groups have an affinity for polar molecules, and the most polar of common molecules is water. So when the object gets wet, water molecules prise loose the odour molecules, cocooning them in tiny mobile parcels of water. For good or ill many escape into the air, reaching nearby noses in vast numbers.

Accordingly, a powerful deodorising strategy is to release other molecules that immobilise pong molecules by binding them with complementary charged groups. Chlorophyll combats smells partly by presenting a metal atom that binds the active groups of many smell molecules. Similarly, by binding key molecules, partly oxidised paraffin wax vapour from the smoke of burning candles also helps clear a room of the stench of cigarettes.

Jon Richfield

Somerset West, South Africa


Blow hole

What is the purpose of the small hole halfway down the outside shell of a Bic ballpoint pen?

Frank Horseman

Derby, UK

If the inside of a ballpoint pen were entirely airtight, the pressure inside it would fall as the ink was used up. This would slow or stop the flow of ink because the higher air pressure outside the pen would push the ink back in.

The reverse could happen if the pen were heated. This would cause the ink to leak out of the pen (presumably onto your most expensive jacket). The hole is there to allow the air pressures to equalise and prevent these problems.

Simon Langley

Ilkley, West Yorkshire, UK

Sven Taylor of Riedisheim, France, took the question to the manufacturer’s French factory. Here is the answer he received - Ed.

The hole is to equalise the pressure inside the pen with the pressure outside the pen. These vents, or holes, in the pen barrels, basically help to prevent ink leakage. Approximately 90 per cent of all pens are vented to prevent leakage. Pens that do not have vented caps contain sealed ink systems and must be pressurised.

Société Bic

Communication Department

Clichy, France


Shampoo pooh-pooh

I have just read the label on my shampoo bottle. The list of ingredients is mind-boggling. How on earth did anyone come up with such a complex concoction and what exactly are chemicals such as sodium diethylene-triamine pentamethylene phosphonate and hydroxyisohexyl 3-cyclohexene carboxaldehyde doing to my hair?

Haydn Ford

Hexham, Northumberland, UK

Sodium diethylene-triamine pentamethylene phosphonate is one of several chelating agents present in the shampoo. These are added because they can complex and deactivate metal ions, stabilising the shampoo against any degradation caused by these ions. They also complex the calcium and magnesium ions present in hard water, preventing the shampoo from separating out on the hair. In some special shampoos these agents can be used to remove the copper ions deposited on hair when swimming in chlorinated water, which can give a greenish tinge to fair hair.

Hydroxyisohexyl 3-cyclohexene carboxaldehyde is one of several fragrant materials which will be present in the shampoo. Together these produce a shampoo that smells attractive and leaves the hair with a fragrance created to make the user feel their hair has been cleansed and refreshed.

How does anyone come up with such a complex concoction? Shampoo formulations are not ‘concocted’; each component is there for a purpose. Those developing such formulations are driven by the need to create a product that users will want to keep buying because they believe it is doing good to their hair.

Tom Jackson

Wigton, Cumbria, UK

Believe it or not, most of the ingredients in shampoo are doing nothing to your hair. Only the detergents clean it, while the rest of the ingredients are used to improve the appearance, smell, texture and shelf life of the product. Providing there are no restrictions on the use of an ingredient, manufacturers are free to use just about anything they like in a cosmetic or toiletry.

A typical shampoo is mostly water, containing between 5 and 20 per cent detergent, with shampoo for dry hair containing less detergent than shampoo for greasy hair. The most widely used detergent is sodium lauryl sulphate (SLS), but as this gives a poor lather, sodium laureth sulphate, which produces a stronger foam, or foam boosters such as cocamide DEA or cocamidopropyl betaine, may also be present. Lather plays no part in the detergency process, although it does keep the detergent and any other active ingredients close to your hair and scalp. Thick lather also reinforces the psychological link between the shampoo and perceived cleaning power.

Oils can be added to counteract the drying effect that detergents have on hair and, therefore, emulsifiers and emulsion stabilisers must also be added. The oils can be anything from natural vegetable oils to synthetic silicone polymers such as methicone and dimethicone. These also have a conditioning effect, helping to smooth the cuticle layer of the hair shafts.

In addition to oils, ‘all-in-one’ conditioning shampoos contain ‘film formers’ to distribute the oils, and antistatic agents, such as cationic surfactants or silicones, to reduce the build-up of static charge in your hair.

At least two preservatives are normally present. One must be water-soluble to protect the watery part of the shampoo, and the other oil-soluble to preserve the oils in the emulsion. The paraben family, formaldehyde, glyoxal and the methylchloroisothiazolinone/methylisothiazolinone mixture are all preservatives commonly added to shampoos.

Thickeners are added to adjust the texture and pouring properties of the product, colourants impart the required colour, UV-absorbers stop the colours from fading, opacifiers give the shampoo a creamy or pearlescent appearance, and natural or artificial fragrances make it smell nice. Shampoo for greasy hair might have a citrus fragrance because of the psychological link between lemon juice and grease-cleansing ability.

Specialised shampoos for dandruff treatment commonly contain zinc pyrithione, and shampoos for treating head lice or fleas contain insecticides. Shampoos to protect dyed hair from the bleaching effect of chlorine and chlorine-liberating compounds in swimming pools contain mild reducing agents, such as sodium thiosulphate, sodium sulphite or sodium nitrite. Once hair colour has faded, however, these shampoos cannot restore it.

Finally, the questioner’s hydroxyisohexyl 3-cyclohexene carboxaldehyde, which also goes by the trade name Lyral, is an artificial fragrance, often used to mask unpleasant odours. Because it is a known contact allergen and can cause sensitisation, its use in cosmetics is regulated within the European Union.

Steve Antczak

Co-author of Cosmetics Unmasked: Your family guide to safe cosmetics and allergy-free toiletries

Lymington, Hampshire, UK


Round the twist

I have often observed defects, or knots, in helical (or spiral) telephone cords. It takes a considerable effort to untangle the cord, which seems to be the only way to restore uniform helicity. How do these defects happen and how can they occur spontaneously during normal handling of the telephone?

Jan Stumpel

Tokyo, Japan

The telephone cord is ‘set’ into a helical form during manufacture. Despite its structure being twisted, the cord is stable in that configuration. When the user rotates the handset in the opposite way to the direction of the helix, some of the in-built twist is removed. However, the de-twisted cord does not become straight, but forms a length of helix in the opposite sense. The greater the reverse rotation of the handset, the greater the length of reverse helix. At the point where the two helixes meet, there is a short region where the twist direction reverses. This is the apparently hooked section which is irritating to telephone users. If one rotates the handset in the direction of the original helix, until all the reverse-twist has been removed, the reversal will soon disappear.

This phenomenon can be seen in yarns whose characteristics are modified in the false-twist textile process, which produces continuous-filament yarn. Twist is inserted into a bundle of filaments, the helical form being stabilised by heating. The twist is then removed. The filaments do not become straight; instead they form helixes of rapidly reversing sense, much like the telephone cord in question. The helixes give the yarn its characteristic stretch properties. Unlike the single telephone cord, textile yarns are usually multi-filament. The detail is therefore rather more complex, but the principle is the same. You can see the same effect under a microscope using an old pair of stretch tights.

Graham Waters

Pontypool, Gwent, UK

Knots in telephone cords are born as metastable knot-antiknot (k-a) pairs when the cord is locally twisted against its natural coil. With a little practice you can resolve a k-a pair into its components, and propagate one element (say the antiknot) to the end of a new cord, where it can be annihilated by twisting the handset. This adds a global twist to the lead and leaves a stable knot in the middle of the cord. In normal use, dormant global twists (gts) are induced by random movements of the handset. They are barely detectable against the natural background, since their only effect is to change the torsional energy of the cable and the total number of turns from end to end. The chance encounter of a dormant gt with a k-a pair (formed by an idle hand) then generates an apparently spontaneous and very stable knot. On a larger scale, you might create an entire observable universe by annihilating spontaneous antiparticles. God doesn’t play dice, He fiddles with phone cords.

Alan Calverd

Bishop’s Stortford, Hertfordshire, UK

The effect occurs spontaneously in my household because the receiver undergoes a half twist every time the phone is answered and a further half twist when the receiver is replaced - thus gradually uncoiling the wire until it can stand no more. Perhaps this is just another example of parity violation - some phones change helicity, others (like mine) don’t. This could be a ground-breaking area in physics research - if only we could find a macroscopic equivalent of the top quark or tau neutrino!

Mark Burbidge

Birmingham, UK


On the line

How long a line could you draw with a single pencil?

Sulin Milne

Newcastle Emlyn, Dyfed, UK

Forget string - we now have a new saying, ‘How long is a pencil line?’ - Ed.

My mind boggled at the range of variables implied by the question. As a scientist turned engineer (now retired), I decided to conduct a simple experiment in which the variables were reduced to a manageable number.

Selecting a clutch pencil with a 0.9-millimetre diameter, and using the manufacturer’s H lead, I drew 100 lines each 30 centimetres long on high-quality printer paper. The pencil was tilted at 75 degrees to the plane of the paper. By measuring the reduction in the length of the refill and allowing for clutchclamping wastage, I concluded that 541 metres of line could be drawn with a lead 60 millimetres long.

I also found by inspection that small changes in some variables had a large effect on the rate at which the lead was used up.

Peter Peters

Sherborne, Dorset, UK

Taking the simple case of a clutch pencil, I found by experiment that a 1-millimetre length of 0.5-millimetre 2B lead would draw about 9 metres of uniform line on ordinary photocopier paper. In my clutch pencil a new lead has a usable length of 50 millimetres, so that’s 450 metres of line per lead. Looking at it another way, it’s easy to work out that 1 cubic millimetre of pencil lead is needed to draw 45.84 metres of line.

A brand-new wooden pencil from a reputable maker is 175 millimetres long with a lead diameter of 2 millimetres. Assuming it is possible to use all but the last 20 millimetres of the lead, and (crudely) that each millimetre of lead draws 9 metres of line as with the clutch pencil, that would give us 1,395 metres of line for the whole pencil.

However, the volume of usable lead in the pencil, assuming again that the last 20 millimetres can’t be used and that half is lost to sharpening, is 243.5 cubic millimetres. At the same volumetric wear rate as in the clutch pencil, that should produce 11,162 metres of line. I expect the actual output will be somewhere between these two answers.

The hardness of the lead will make a difference, as will paper type, the density of the line and how careful the user is not to sharpen too often or too far.

Andrew Fogg

Sandy, Bedfordshire, UK


Field of bubbles

When I placed a glass of still mineral water in front of my computer screen I noticed that tiny bubbles started to form around the edge of the glass. Why does this happen and is the water still OK for me to drink? And if it was the screen that caused this, what is happening to my body, which is essentially liquid, when I sit in front of my computer?

C. Strange

Lancashire, UK

The correspondent who claims that bubbles form in mineral water as a result of him placing a glass of water in front of a computer screen is mistaken about the cause.

I always have a glass of water by my side. If I leave that glass for an hour or two, without touching it or drinking from it, small bubbles form over the glass surface below the water level.

I presume in your correspondent’s case that the mineral water was bottled from a pressurised source where more air was dissolved than would be if it was bottled at atmospheric pressure. In my case, the bubbles come from the air dissolved due to the mains pressure in the pipes. The water is perfectly safe to drink, including its bubbles.

Gordon Thompson

Crewe, Cheshire, UK

Water has air dissolved in it, and the colder it is the more air is dissolved. Fish tanks found in aquariums and homes make use of this dissolved air to allow the fish to breathe. They must also have some means of refreshing the air content of the water, or the fish will suffocate and die.

Your glass of mineral water was probably chilled when you poured it out so a lot of air will have been dissolved in it. As it warmed up, air came out of solution and formed bubbles on the side of the glass. Being in front of the monitor may have warmed the water a bit more quickly, but those bubbles were going to appear anyway.

Stephen Forbes

Leeds, UK


Humming words

Electricity is supposed to be the ‘silent servant’. So why do transformers hum?

D. J. Priestley

University of Wales, Swansea, UK

Transformer hum is caused by a phenomenon known as magnetostriction. To understand why, it is necessary to take a look at how transformers work.

Inside they contain two coils of wire, the primary and the secondary coils, wound onto opposite sides of a ring made out of many thin sheets of iron or some other ferromagnetic material.

An alternating current flowing through the primary coil generates an alternating magnetic field in the iron ring, which in turn creates a voltage in the secondary coil. The ratio of the primary voltage to the secondary voltage is equal to the ratio of the number of turns of wire in the primary coil to the number of turns in the secondary. This allows us to change the hundreds of thousands of volts running through overhead power lines to a voltage low enough to be safe to use in our homes.

The iron making up the ring that joins the primary and secondary coils is divided into microscopic domains. In each of these domains, the magnetic field points haphazardly in different directions, much like a classroom full of unruly pupils who are running all over the place.

However, when the iron is placed in an external magnetic field, these domains tend to line up and add together, producing a strong magnetic field pointing in one direction, just as schoolchildren will snap to attention at a teacher’s command.

As the domains line up, the material very slightly changes its length to accommodate the rearrangement. This is magnetostriction. As the magnetic field through the iron alternates, the iron expands and contracts over and over again. These vibrations produce the sound waves that create the transformer’s distinctive hum.

In the US, the mains voltage alternates 60 times every second (60 hertz), so the material expands and contracts 120 times per second, producing notes at 120 Hz and its harmonics. In Europe, where the mains supply is 50 Hz, the hum is nearer 100 Hz and its harmonics.

Michelangelo D’Agostino

Berkeley, California, US

In addition to magnetostriction there are two other reasons why transformers tend to emit sound.

The first is imperfect insulation. Just as the corona discharge from power lines in damp air produces a buzzing sound, insulation breakdown in a transformer can also be noisy. In practice, however, insulation breakdown usually occurs deep inside a transformer, where the heat stress is most severe, and no audible noise emerges until the final catastrophic failure.

The second is caused by moving parts. Power supplies such as those you find behind computers sometimes make a buzzing sound, which is most likely to be the wire winding moving as the transformer’s magnetic field and the current passing through it act together to produce a force similar to that in an electric motor. On the face of it, it seems that eventually metal fatigue ought to set in, but in practice transformers seem to be able to keep buzzing for years.

Other parts of a transformer can also buzz. For example, if the clamps that hold the parts together are not fixed tightly, they can rattle inside the casing.

David Billin

Carshalton, Surrey, UK

The description ‘silent servant’ did not really mean that electricity was silent. The expression was coined in the early 1920s by the General Electric Company in the US and used in advertisements and popular magazine articles to promote the use of electrical equipment in homes. The idea it was meant to convey was that electricity, unlike humans, could perform tasks without speaking or being spoken to, not that electricity itself was silent. Indeed, many pieces of electrical apparatus were, and still are, quite noisy.

Michael Brady

Asker, Norway

Silent transformers do exist and have been around since the early 1980s. The first models were too heavy and bulky for many types of equipment. But in modern appliances, ‘switchmode’ power supplies are used which have much smaller transformers supplied by alternating current at a frequency too high for humans to hear, with sharp-edged pulses rather than smooth signals.

The mains AC frequency of 50 or 60 hertz - which is noisy when passed through a transformer - is increased to the higher frequency, usually via an oscillator. The current then enters one or more transformers that step down the voltage and, thanks to the inaudible nature of the higher frequency, allow the transformer to perform quietly.

The more rapidly changing magnetic field allows smaller transformers to be used. So as well as converting the audible humming sounds to inaudible, ultrasonic whistles, it helps make equipment smaller and lighter.

Patrick McTiernan

Swindon, Wiltshire, UK


Let’s twist again

Given that the average person twists and turns up to 100 times during a night’s sleep, why is it so unusual for anyone to fall out of bed? Does the human brain have a built-in warning system that is triggered when one’s body goes near or over the edge?

G. Rowlands

Purley, Surrey, UK

Some 25 years ago, at the University of Edinburgh, Geoffrey Walsh and I investigated the reasons why adults do not usually fall out of bed while asleep. Since no one can know what movements they carry out during sleep unless some form of recording is used, we devised a simple experiment.

Volunteers slept on a very wide mattress in a warm room, with no coverlet so that they would not be able to detect in their sleep where they were in the bed. Their head position was noted from a choice of four positions: nose to left, nose up, nose to right, or nose down. The apparatus was unsophisticated, and comprised a rugby scrum cap onto which I stitched a circle of plastic tubing complete with a short piece of glass tubing. The tubing contained some mercury, and I thrust some needles through the tubing wall at suitable points and attached a dry battery so that small voltages were generated according to the head position. These were recorded all night on an electroencephalographic recorder.

Periods of sleep and waking were recorded by arranging for a small sound to be made at around 10-minute intervals. If the volunteer was awake and heard it, they pressed a bell push attached to their clothing. This allowed us to discount movements made during this time. During sleep, of course, no response was recorded.

Participants turned at irregular intervals throughout their sleep, for example, nose to left, nose up, nose to right, then back again. But they never turned nose down. As a result, they did not roll over and over so that they would fall out of bed. Instead, they remained in roughly the same position all night.

At what age does this behaviour appear? Because we could not leave young children alone with such fascinating apparatus on their heads, I simply watched my niece and nephew, aged about four and two, in their cots over a period of some six hours while they slept. During the night they did turn nose down from time to time. So they could turn over and over, and could have fallen out if their cot sides had not stopped them.

I concluded that quite early in life we learn that it is difficult to breathe if we turn nose down, and we avoid it even when asleep. As a result we won’t roll out of bed.

John Forrester

Edinburgh, UK

If you find not falling out of bed in your sleep an impressive skill, spare a thought for sailors. In some ships they still sleep in hammocks; and the naval version, called a mick, is slung tight and level. Though it does seem to reduce sensitivity to the ship’s motion, any sleeper who cannot lie flat and still in a mick is a hostage to fortune. It dumps you instantly if you so much as breathe asymmetrically, and yet thousands of sailors have slept soundly in them for centuries.

Antony David

London, UK

The ability of people to adapt to unfamiliar sleeping situations (from broad beds to narrow beds to hammocks to futons spread on a floor) suggests that on top of the processes investigated by John Forrester and Geoffrey Walsh we are somehow able to tell ourselves how much we can move before we fall asleep. In some ways this is similar to telling ourselves what time to wake up, which many people can do without an alarm clock - Ed.


Guilty as charged

As back-up for my digital camera, I fully charged a set of AA nickel-metal hydride (NiMH) batteries, and carried them in a battery box with no chance of accidental connection. When I needed them some time later, they had completely discharged. Do rechargeable batteries leak their charge over time? If so, why, and how long does it take? For extra back-up, I now carry a set of ordinary alkaline batteries as well.

Joseph Oldaker

Nuneaton, Warwickshire, UK

Nickel-metal hydride batteries have a high rate of selfdischarge - about 30 per cent per month. This means that every two months their charge diminishes by a factor of 2, and in a year they will discharge to about 1.4 per cent of their full charge - effectively dead.

Nickel-cadmium batteries are somewhat better, and lithium batteries are much better at only about 2 or 3 per cent discharge per month. In 2005, low self-discharge NiMH batteries were introduced. These are sold as pre-charged or ready-to-use. They are more expensive than old-fashioned NiMH batteries but can be used in many applications, such as a clock, where normal NiMH batteries would be unsuitable - though in some of these applications it might be more economical just to use non-rechargeable batteries.

Eric Kvaalen

La Courneuve, France