Why Don't Penguins' Feet Freeze?: And 114 Other Questions - Mick O'Hare (2009)

Chapter 5. Domestic science

8986 Breaking the mould

What is the name of the dreaded black mould that colonises damp places in bathrooms? Because materials produced to remove the mould do not seem to work, and nor do household bleaches, detergents and solvents, can anyone suggest a remedy other than abrasives?

G. W. Green

Malvern, Worcestershire, UK

The infamous black mould is the fungus Aspergillus niger. The reason it seems so difficult to eradicate is that the visible black manifestation is merely the exposed structure of the fungus, which is mainly comprised of the fruiting bodies. In addition to this visible material, there is invariably an insidious network of hyphae or mycelia which lie in the substrate of wallpaper or plaster, and feed on the minerals contained within.

Eradication of the mould requires not only the repeated physical removal of the visible growth but the simultaneous use of a penetrative fungicide capable of permeating the substrate and killing off the unseen root structure. The analogy is that of trying to eradicate ground elder or horsetail from your vegetable patch by merely strimming the visible plantlets.

Andrew Philpotts

Hexham, Northumberland, UK

The Aspergillus fungus has been a constant source of annoyance in local council accommodation throughout the nation. It is prevalent where cool, still air deposits condensation next to steel window frames, concrete-screed ceilings, water-tank enclosures and similar areas.

Current medical opinion is that this fungus is a major source of allergenic disease and that it produces carcinogenic aerosols, so removal of the unsightly nuisance is also important for health.

I have experienced problems when attempting to remove Aspergillus. Table salt and bleach have only limited success, but I finally effected a permanent solution by washing the affected areas several times with a systemic fungicide available from any garden store. This may not, however, be the safest solution as the fungicide may be as toxic as the fungus.

Glyn Davies

Kingston, Surrey, UK

The previous answers seem to have fallen into the trap of assuming that any mould that is black is Aspergillus niger. In surveys of Scottish housing carried out by my laboratory, the incidence of this species has been rather low.

The most common dark mould in growths on bathroom and other damp walls are likely to be species of Cladosporium, with Aureobasidium, Phoma and Ulocladium thrown in for good measure. Even green species of Aspergillus and Penicillium can look black when soaked.

The situation is much the same in mainland Europe, so it is likely that it will not be very different in Northumberland or Surrey, unless the bathrooms there come closer than most British bathrooms to providing the subtropical and tropical climates that favour A. niger.

A really black fungus in about 15 per cent of houses in Scotland with mould problems is Stachybotrys aira. Wallpaper, jute carpet backing or the cardboard wrapper of gypsum board all provide ideal cellulosic substrates on which it can thrive in very damp conditions. This type of mould may present the greatest hazard of any to the health of occupants of mouldy buildings. Its airborne spores are allergenic and powerfully toxicogenic. Its toxins inhibit protein synthesis, and are immunosuppressive, an irritant and haemorrhagic.

It is well known that fodder contaminated by Stachybotrys can kill horses, and it is also harmful to the stable hands. Currently, this mould is of particular concern in North America, where it has been implicated in episodes of building-related illness ranging from chronic fatigue syndrome in adults to fatal pulmonary haemosiderosis in infants. Consequently it has been the subject of lawsuits (one for the sum of $40 million) taken out against builders and employers.

Brian Flannigan

Department of Biological Sciences

Heriot-Watt University, Edinburgh, UK

The outside wall of my bathroom in Pimlico used to grow a superb crop of mould which removed the wallpaper and infested the plaster. To remove this Aspergillus, I used a single washing-down with a dark pink solution of potassium permanganate crystals which effected a cure with no recurrence.

Bill Christie

Fairlight Cove, East Sussex, UK

Readers should note that potassium permanganate is poisonous if swallowed – Ed.

Household bleach does not remove the staining caused by Aspergillus niger. But spraying the surface or painting it with an aqueous solution of 10 per cent zinc sulphate prevents the re-emergence of the fungus as long as the molecules of zinc sulphate are not washed off.

Farrokh Hassib

London, UK

8986 Hot stuff

Is it true that hot water placed in a freezer freezes faster than cold water? And if so why does this happen?

Ian Popay

Hamilton, New Zealand

This question was raised many years ago in New Scientist and never answered satisfactorily. This time we are closer to settling the controversy with answers from several people who have tried the right experiments. Counter-intuitive though it may be, it does appear that hot water can freeze more quickly in a refrigerator. Better thermal contact if the water container is placed into an iced-up freezer compartment and a different pattern of convection currents which allow hot water to freeze faster seem the best explanations. Which effect predominates depends on the fridge, the container and where it is placed – Ed.

The questioner is correct – it is possible to produce ice cubes more quickly by using initially hot water instead of cold. The effect can be achieved when the container holding the water is placed on a surface of frost or ice. The higher temperature slightly melts the icy surface on which the container rests, greatly improving the thermal contact between the container and the cold surface. The increased rate of heat transfer from the container and contents more than offsets the greater amount of heat that has to be removed. The effect cannot be obtained if the container is suspended or rests on a dry surface.

This effect was first noted by Sir Francis Bacon using wooden pails on ice. My own investigation showed ice cubes could be obtained within 15 minutes rather than 20 minutes if the frost in the refrigerator was deep enough. The incentive to get your ice a little quicker is obviously greater in Australia than in cooler countries.

Michael Davies

University of Tasmania, Australia

But Sir Francis Bacon was not the first to note the effect. Aristotle’s account in Meteorology below implies a similar explanation: ‘Many people, when they want to cool water quickly begin by putting it in the sun. So the inhabitants when they encamp on the ice to fish (they cut a hole in the ice and then fish) pour warm water round their rods that it may freeze the quicker; for they use ice like lead to fix the rods.’

David Edge

Hatton, Derbyshire, UK

And it seems untrue that the ‘effect cannot be obtained if the container is suspended or rests on a dry surface’ . . .

This question was raised in New Scientist in 1969, by a Tanzanian student named Erasto Mpemba. He discovered that ice-cream mixture froze more quickly when put in the freezer hot than if allowed to cool to room temperature first. I got the same sceptical comments from my teachers as Mpemba did when I based my sixth-form project on his question.

First, the project showed that water, either from the tap or distilled, behaved in the same way as ice-cream mixture; the chemical composition is not important. Second, it demonstrated that a reduction in volume by evaporation from hot water was not the cause. Placing thermocouples into the water showed that water at about 10 °C reached freezing point more quickly than water at about 30 °C, as predicted by Newton’s law of cooling, but that thereafter, water that started off warm solidified more quickly.

In fact, the maximum time taken for water to solidify in the freezer occurred with an initial temperature of about 5 °C, and the shortest time at about 35 °C. This paradoxical behaviour can be explained by a vertical temperature gradient in the water. The rate of heat loss from the upper surface is proportional to the temperature. If the surface can be kept at a higher temperature than the bulk of the liquid, then the rate of heat loss will be greater than from water with the same average temperature, uniformly distributed. If the water is in a tall metal can rather than in a flat dish, the paradoxical effect disappears. We argued that temperature gradients in the tall can were short-circuited by heat conduction through its metal walls.

The question has certainly made me reluctant to take accepted wisdom for granted when it comes to observations which do not fit preconceived notions of what is correct.

J. Neil Cape

Penicuik, Midlothian, UK

The classic experiment uses two metal buckets placed in the open air on a cold, preferably windy, night. Stationary water is a poor conductor of heat and ice forms on the top and around the sides. If the initial temperature is around 10 °C, cooling of the core is very slow, particularly as loose ice floats to the top, inhibiting normal convection. There is no means by which the warmer water can come into contact with the cold bucket and transfer its energy to the outside.

If the initial temperature is about 40 °C, strong convection is established before any water freezes, and the entire mass cools rapidly and homogeneously. Even though the first ice forms later, complete solidification of the hot water can occur more quickly than if the water starts off cold.

The conditions are critical. Obviously, if the cold bucket starts at 0.1 °C and the hot at 99.9 °C the experiment is unlikely to cause surprise. The containers must be large enough to sustain convection with a small temperature gradient, but small enough to extract heat quickly from the bucket’s surfaces. Forced air cooling on a windy night helps.

It is difficult to generate suitable conditions in a domestic freezer but the anomaly can be demonstrated in an industrial chiller or a laboratory environmental chamber.

Alan Calverd

Bishop’s Stortford, Hertfordshire, UK

It’s true and I have verified the assertion by experiment. The only limitation is that the container of water must be relatively small so that the capacity of the freezer to conduct away the heat content is not a limiting factor.

Cold water forms its first ice as floating skin, which impedes further convective heat transfer to the surface. Hot water forms ice over the sides and bottom of the container, and the surface remains liquid and relatively hot, allowing radiant heat loss to continue at a higher rate. The large temperature difference drives a vigorous convective circulation which continues to pump heat to the surface, even after most of the water has become frozen.

Tom Hering

Kegworth, Leicestershire, UK

This is a cultural myth. Hot water will not freeze faster than cold water in the freezer. However, hot water cooled to room temperature will freeze faster than water that has never been heated. This is because heating causes the water to release dissolved gases (mostly nitrogen and oxygen) which otherwise reduce the rate of ice crystal growth.

Tom Trull

University of Tasmania, Australia

Sceptical Tom Trull from the University of Tasmania might like to stroll over to the refrigerator of the first letter-writer, Michael Davies, also from the University of Tasmania. The experimental evidence suggests that the effect is real– the absence of dissolved gas could be another factor that speeds crystal growth.

And there could be yet another factor that none of our letter-writers have described – supercooling. More recent research shows that because water may freeze at a variety of temperatures, hot water may begin freezing before cold. But whether it will completely freeze first may be a different matter – Ed.

In scientifically controlled experiments this effect seems to be real. We assume that the temperature in the freezer stays constant during the freezing process, as do the variables of the samples such as container size, conduction and convection properties inside and outside the container.

However, I feel that one more variable is present and that is an overlooked temperature variation in the freezer. The temperature oscillation inside the freezer depends on the sensitivity of the thermoelement and the timer of the controller system. We may assume that at the freezer’s standard temperature the power used for cooling the freezer operates at a standard rate. If a bucket of cold water is added, it may produce only a small effect on this power output as it will not trigger the temperature sensor. However, a bucket of hot water may easily activate the sensor and release a short but powerful cooling of the freezer with a cooling overshoot, depending on the timer.

This may be overlooked by an observer at home. I have seen a similar effect in an electric sauna. By fooling the temperature sensor by splashing water I increased the oven’s output.

Matti Jarvilehto

University of Oulu, Finland

Recent, as yet uncorroborated, research from the University of Washington at St Louis, US, has offered yet another possibility. Solutes, such as calcium and magnesium bicarbonate, precipitate out if water is heated. These can be seen inside any kettle used to boil hard water. However, water that has not been heated still contains these solutes and as it is frozen the ice crystals that are forming expel the solutes into the surrounding water. As their concentration increases in the water that has yet to solidify they lower its freezing point like salt sprinkled on a road in winter. This water therefore has to cool further before it freezes. Additionally, because the lowering of the freezing point reduces the temperature difference between the liquid and its surroundings, the heat loss from the water is far less rapid – Ed.

8986 Stick with it

Why doesn’t superglue stick to the inside of its tube?

Ajit Vesudevan

Oxford, UK

Superglue will not stick to the inside of its tube because the tube contains oxygen in the form of air but excludes water. Oxygen inhibits whereas water catalyses.

Yvonne Adam

Bostik Limited

Leicester, UK

Superglue doesn’t stick to the inside of the tube because, being based on a cyano-acrylate monomer, it requires moisture in the form of water or some other active hydrogen-bearing compound to polymerise.

This explains why the best join between two surfaces is made using a thin glue line. An excess thickness of glue will lead to a retarded cure. This moisture sensitivity explains two things. First, why the bottle comes with a seal that’s impossible to break without covering oneself in glue and why the resulting spillage adheres so well to your skin – being warm and moist, skin makes an ideal substrate.

Brian Goodliffe

Wetherby, West Yorkshire, UK

The Loctite company in the US discovered the inhibition by oxygen of the otherwise rapid polymerisation of cyano-acrylate. That is why the bottle must always be left with plenty of air inside. The liquid monomer converts to solid polymer when oxygen is excluded by trapping it between close-fitting surfaces.

E. Barraclough

Otterham, Cornwall, UK

8986 Smell from hell

Why is it that, whatever they contain, dustbins always smell the same?

Rodri Protheroe

Colchester, Essex, UK

The source of the smell is most probably caused by bacteria and fungi feeding on the organic matter in the rubbish. It will be most noticeable if the bin is in a warm and damp place.

The smell will not always be exactly the same, but it will be more characteristic of the different organisms than of the type of food they consume. The smell you get from penicillin mould growing on an orange is just the same as that from penicillin mould grown in a laboratory culture. It is pungent, characteristic and very common.

Analyses of household rubbish have detected very pathogenic bacteria, including Pasteurella pestis, the bacterium responsible for causing bubonic plague. So don’t sniff too hard.

Cary O’Donnell

Welwyn, Hertfordshire, UK

I was pondering this question while taking out rubbish and I realised that dustbins do not smell the same. A bag containing foodstuffs will inevitably be ripped open by local cats unless protected by a dustbin, but a bag without food is not. It would seem that, although the bags smell similar to humans, they are noticeably different to cats.

As for why they smell similar, that would be because they invariably contain similar objects. However, garden refuse, for example, smells nothing like kitchen refuse, which in turn smells nothing like bathroom refuse.

Stewart Ravenhall

Newport Pagnell, Buckinghamshire, UK

8986 Sticky problem

Why does sticky tape when pulled from a roll quickly (at 10 millimetres a second) become almost transparent, but when pulled off slowly (at 1 millimetre a second) become opaque? Indeed, if while pulling tape off quickly, one pauses for a few seconds, a distinct line is left on the otherwise transparent tape. Can anyone explain?

David Holland

Broadstone, Dorset, UK

The reason for the difference in behaviour lies in the response of the adhesive layer on the tape to the rate at which it is stressed. When the tape is peeled back slowly, the adhesive responds by forming long drawn-out strands between the two pieces of tape which break and fall back onto the tape, forming an opaque, rippled surface. These strands can be seen with the naked eye or with a hand lens.

When pulling off at higher speeds, instead of being able to stretch, the incipient adhesive strands formed break at much lower elongations and produce much less disturbance of the adhesive layer.

The difference arises because of the viscoelastic nature of the polymer which forms the sticky material. The material has a viscous component giving it some of the physical properties of treacle. It also has an elastic component which causes it to behave like a solid material, such as metal in the form of a wire. When treacle is stretched it forms long strands almost never breaking, whereas metal wire has a comparatively low elongation and breaks when pulled. At low pulling rates, the adhesive is more like treacle and at high rates, more like the metal wire.

Ultimately, the behaviour is dependent on the time of relaxation processes at the molecular level. Since time is in some sense equivalent to temperature when considering molecular movements, it is interesting to cool the tape in a freezer. Now, pulling at the lower speed produces a much more transparent region. Because there is not enough time for the long-chain molecules to unravel in long strands, the adhesive breaks in a brittle manner.

Stephen Hancock

Stockport, Cheshire, UK

8986 Kettle hums

Why does a kettle sing? Why does the note rise at first, then fade for a while and then return with a falling frequency?

Don Munro

University of Newcastle

New South Wales, Australia

If you leave the lid off your electric kettle and switch on, you can see what is happening. The heating element quickly becomes covered with small silvery bubbles, each about 1 millimetre in diameter. These are air bubbles, forced out of solution by heat from the element. Rough parts of the element’s metal surface provide nuclei for their growth and they eventually detach from the hot element and rise to the surface. These bubbles form and burst silently, and are clearly not the cause of the kettle singing.

After about a minute, the air bubbles are replaced by innumerable smaller bubbles of superheated steam that cling to the growth nuclei on the heating element.

A few seconds later, these primary steam bubbles become unstable. As each bubble forms, its buoyancy tends to pull it away from the hot surface. Being surrounded by water which is still far below boiling point, the primary steam bubble suddenly condenses, collapsing implosively. Curiously, the bubble does not vanish completely, but leaves behind a minute secondary bubble, presumably of water vapour, that does not immediately condense but is whirled away by the convection currents. Soon there is such a cloud of these secondary bubbles that the water becomes turbid for half a minute or so.

Meanwhile, the shock waves transmitted through the water by the imploding primary bubbles produce a sizzling sound. You can give this sound a more definite pitch by temporarily replacing the kettle lid. This defines a volume of air above the water surface that resonates to some of the frequencies present in the shock waves.

Soon, the cloud of secondary bubbles clears, and there is a general increase in the size of the primary steam bubbles that are still forming on the element. These are no longer forced to collapse immediately and implosively, since the surrounding water is now practically at boiling point, so the noise fades away. As they grow, streams of buoyant primary bubbles detach themselves from the surface of the element, condensing in the cooler water a centimetre or so above it.

Within seconds, the water becomes hot enough to allow large detached primary bubbles to reach the surface, and now you can hear only the return of sound with the low gurgle of their bursting in the air cavity above the water.

Roger Kersey

Nutley, East Sussex, UK

8986 Key problem

Why are the rows on a calculator or number keypad arranged with the lowest numbers at the bottom, when we normally read from the top downwards? And why are telephone keypads arranged the other way, with the lowest numbers at the top?

M. D. Berkson

Bishop’s Stortford, Hertfordshire, UK

Mechanical adding machines, based on rotating wheels, always have the 0 button adjacent to the 1 button. By convention, most old adding machines had the numbers increasing in value from the bottom and this may be a hangover from when the machines had levers on the wheels rather than buttons. When the numbers were put on to a pad arranged as a three by three grid with one left over, the order of the numbers, as far as possible, was kept the same.

On a rotary telephone dial the 0 comes adjacent to the 9 because a 0 in the telephone number is signalled by 10 pulses on the line. When telephones acquired push buttons in a grid the ordering of the buttons was carried over from the old telephone dial.

Nicko van Someren

Cambridge, UK

8986 Mirror image

Why is an image in a mirror inverted left to right but not top to bottom?

Kishor Bhagwati

Lausanne, Switzerland

The mirror does not reverse images from left to right, it reverses them from front to back relative to the front of the mirror. Stand facing a mirror. Point to one side. You and your mirror image are pointing in the same direction. Point to the front. Your mirror image is pointing in the opposite direction to you. Point upwards. You both point in the same direction. Now stand sideways on to the mirror and repeat. You are now pointing in opposite directions when you point sideways. Place the mirror on the floor and stand on it. This time you point in opposite directions when you point upwards and your upside down image points downwards. In all cases the direction reverses only when you point towards or away from the mirror.

Hilary Johnson

Malvern, Worcestershire, UK

The answer stems from the fact that a reflection is not the same as a rotation. Our bodies have a strong left-right symmetry, and we try to interpret the reflection as a rotation about a central vertical axis. We imagine the world in front of the mirror has been rotated through 180 degree about the mirror’s vertical axis, and it has arrived behind the mirror where we see the image. Such a rotation would put the head and feet where we expect them, but leave the left and right sides of the body on opposite sides to where they appear in the reflection.

But if instead we imagine the world to have been rotated about a horizontal axis running across the mirror, this would leave you standing on your head, but would keep the left and right sides of your body in the expected positions. The image would then appear top/bottom inverted, but not left-right.

So whether you see the image as left-right inverted or top-bottom inverted – or for that matter inverted about any other axis – depends upon which axis you unconsciously (and erroneously) imagine the world has been rotated about.

If you lie on the floor in front of a mirror you can observe both effects at once. The room appears left-right reflected about its vertical axis, while you interpret your body as being left-right reflected about a horizontal axis running from head to foot.

Peter Russell

London, UK

Actually, a mirror does not invert at all. Look at your face in a mirror: the left side appears on the left and the right on the right.

Now look at someone else’s face without a mirror. It has been inverted because of the rotation necessary to turn and look at you: their right side is on your left. They could equally well turn to look at you by standing on their head, in which case you see their left on your left, but now the top of their head appears at the bottom. We don’t normally do this because it’s not very comfortable.

Try this experiment. Write a word on a piece of paper and hold it up to a mirror. You automatically rotate it about a vertical axis and it appears in the mirror inverted left to right. It is this rotation which inverts the image, not the mirror.

Try the experiment again, and this time when you hold the paper up to the mirror, rotate it about a horizontal axis. The word will be inverted top to bottom.

Alan Harding

Stansted, Essex, UK

The problem is caused by the way we visualise the mirror image. We imagine ourselves standing on a carousel, which has done a half turn to put us where we see the image – that is, in the mirror. We see that the top and bottom of our bodies in the mirror image are in the same place, but left and right are reversed.

If instead of a carousel we used a ferris wheel to rotate ourselves, and imagined ourselves strapped upright in the seat we would see a different result. When the wheel does a half turn, the mirror image now has left and right in the correct places, but top and bottom are reversed.

The trouble is that we are incorrectly using rotation for these experiments, when, in reality, the mirror reflects front-to-back. Because this is a difficult thing to do with our body, we mentally substitute rotation, which doesn’t quite fit what we see.

Generally, we prefer to keep top and bottom correct, so we see a left to right reversal in a mirror, although we could see top to bottom reversal if we wished.

David Singer

San Francisco, California, US

8986 Sealed in light

When I am opening some types of self-sealing envelopes I notice that there is a purple fluorescent effect within the gum. It only lasts for a very short time, but can be repeated if I reseal the envelope and pull it apart again. What causes this effect?

Stewart Duguid

Edinburgh, UK

The coloured glow is a form of chemiluminescence. Separating the gummed surfaces requires energy that breaks the attractive forces between the molecules of gum.

Presumably, the act of pulling apart the surfaces supplies excess energy to the gum molecules that lifts them into an excited state. As they decay back to their normal state the energy is released in the form of visible light. The difference in energy between the excited and ground states defines the wavelength and hence the colour of the light produced; in this case purple.

This phenomenon is different from fluorescence, where light (often ultraviolet) is absorbed and then re-emitted at a longer wavelength (in the visible spectrum). Fluorescence gives rise to ‘Day-Glo’ colours and the blue glow you might observe while drinking tonic water near one of the ultraviolet lamps often found in nightclubs.

Paul Wright

Peel, Isle of Man

A similar effect can be seen when stripping off a length of electrical insulating tape.

I first noticed this about 30 years ago and the discovery came, by coincidence, shortly after an explosion in a coal mine. The last people to go down the mine before the explosion had been a crew of electricians.

I wondered if the electricians had been using insulating tape and so I wrote to the authorities questioning the possible danger of insulating tape as a source of ignition.

However, I received a reply which stated that the effect was well known, but that there was insufficient energy in the sparks to ignite methane in the mine.

Mike Gay


I noticed the glow mentioned by your previous correspondents – on Royal Society of Chemistry envelopes in my case – and wondered about ignition of flammable atmospheres. As I facetiously pointed out to the society, members of the Royal Society of Chemistry often open envelopes in environments of lower ignition energy than that of methane.

More recently, there has been an explosion attributed to essentially this cause, or at least to peeling off an adhesive label. The entry in future editions of Bretherick’s Handbook of Reactive Chemical Hazards may run something like this:

Adhesive labels

Tolson, P. et al, J. Electrost., 1993, 30, 149

A heavy-duty lead-acid battery exploded when an operator peeled an adhesive label from it. Investigation showed that this could generate >8 kV potential. Discharge through the hydrogen/oxygen headspace consequent upon recharging batteries caused the explosion. The editor has remarked vivid discharges when opening Royal Society of Chemistry self-adhesive envelopes.

P. Urben

Kenilworth, Warwickshire, UK

8986 Jelly roll

Why does a jar containing Swarfega resonate so when bumped?

Bruce Buswell

Bath, Somerset, UK

Swarfega, like many materials, has both a viscous and an elastic side to its nature. It is a gel made up of a network of weak elastic bonds. These are easily broken under the shearing action caused by your fingers when using it to clean your hands. If these bonds are not broken down but are subjected to a force within their elastic limit (such as being bumped in a jar) they will store the energy and oscillate like a spring.

The period of oscillation is related to the bond energy and length. So when large networks of strong and relatively short-range bonds are struck, as in a metal anvil, you get high-pitched ringing tones. Weaker, longer bond networks like those in Swarfega give low-frequency natural harmonic oscillations when struck. This low resonance is quickly damped by the viscous component of Swarfega which, instead of storing the energy of a strike, dissipates it in forms such as heat and entropy.

Wayne Collins

Toddington, Bedfordshire, UK

Swarfega is either a gel or a very viscous liquid (and there may be a phase change within the normal range of room temperatures). It is somewhat unusual in that most common substances of that nature have high internal friction losses, and you normally get a thud by striking the tin. The low internal losses of Swarfega suggest that the substance might, on the molecular scale, have some long-range structural order.

Because it is a detergent, its molecules will have an ionic end, which is attracted to water, and a fatty end, which is repelled by water. These molecules could form roughly spherical associations, with the fatty ends outwards and water in the centre. These would then slide easily over each other when the substance was grossly deformed and could resonate mechanically with low losses if the driving disturbance is of small amplitude. I recall that if some water is added to the Swarfega, the resonating effect is much reduced.

J. M. Woodgate

Rayleigh, Essex, UK

8986 Klingoff

Why doesn’t cling film cling to a metal bowl as well as it does to an equally smooth glass or ceramic one?

Tim Bloomfield

Letchworth, Hertfordshire, UK

Cling film, known as cling wrap in the US, works because it acquires an electric charge as it is peeled from the roll. It can then stick to an insulating body by the same mechanism that an uncharged piece of paper sticks to the charged glass of your computer or television screen.

The mechanism relies upon the cling film and the object to which it is sticking being at a substantially different electrical potential. This works when the object is an insulator. When the object is metal, the charge on the film is dissipated throughout the object, so negating the effect.

Old cling film taken off the roll doesn’t work either. After a while, the charge breaks away, and the clingyness is lost.

Alistair Hamilton

By email, no address supplied

Cling film becomes charged with static electricity as it peels from the roll. You can sense the charge by peeling some off and holding it near your face – you will feel the hairs on your cheek stand up. Metal drains away static, whereas glass (or plastic) retains static on its surface. The more static, the greater the cling.

Jeffrey Wells

By email, no address supplied

8986 Rustle riddle

What generates the energy that makes thin, white supermarket bags so noisy?

Lucy Birkinshaw

Leicester, UK

The energy is generated mostly by you, because the bag will not rustle by itself. The noise is caused by sharp movements of the kind you get when a stiff plate buckles or gets rubbed. The bags are made of polyethylene film which, untreated, is waxy and floppy and not very noisy. It is elastic rather than plastic, so it absorbs stresses quietly.

However, to make the bags, the film is stretched to get it thin enough to be convenient to handle and cheap enough to give away with the goods. This partly aligns its molecules into stiffer sheets. To make the bags look better and the contents more anonymous, manufacturers add fillers for colouring and further stiffening. The result is a bag which audibly protests every crinkle, crumple and abrasion.

Jon Richfield

Dennesig, South Africa

8986 First light

Why do light-bulb filaments usually blow when first switched on, and not at the end of a long evening’s use when they are at their hottest and most hard working?

Alan Staten

St Ives, Cornwall, UK

When a light bulb is switched on, its delicate filament is hit with a triple whammy.

The resistance of the metal filament rises with its temperature. When it is switched on, its resistance is less than a tenth of its usual working level so an initial current more than 10 times the rated value surges into the filament, heating it very rapidly and creating thermal stress.

If any part of that filament is slightly thinner than the rest, this area will heat up more quickly. Its resistance per millimetre will be higher than that of the rest of the filament, so more heat will be generated along this stretch than in the adjacent parts of the filament, amplifying the thermal stress.

In addition to all this, the filament is wound into a coil which also acts as an electromagnet. Because of this magnetism, each turn of the coil deflects its neighbouring turns so that the initial surge of current jolts the thin and delicate filament, creating mechanical stress.

So it is no wonder the poor thing goes splat when you throw the light switch.

Robert Senior

Uppingham, Rutland, UK

The more electric current that passes through the tungsten-type metal filament of a conventional light bulb, the more the metal heats up. When the bulb is first switched on, its filament goes very quickly from room temperature to white hot. This rapid heating exposes the filament to maximum physical and thermal stress. When the current is switched off, the filament is surrounded by the warm structure of the bulb, so it changes temperature more slowly than when it is switched on. The filament therefore has a much greater chance of blowing when switched on than when it is operating or when it is cooling after being switched off.

Ross H. Clements

North Narrabeen, New South Wales, Australia

Light filaments blow when switched on because that is when the highest current and highest temperatures occur. If you measure the resistance of a light bulb when cold you will find it to be far less than the rated resistance.

In a 100-watt bulb I have just measured, the cold resistance was only 6 ohms, while the hot resistance is about 140 ohms. Thus current flow and heat generated are far higher at switch-on than after the filament has heated up to its rated temperature. This is especially true at places where the filament has thinned due to age and evaporation of the metal. The large initial current heats those parts of the filament far above the standard operating temperature, which melts them. A bulb filament works harder at start-up, and heats the thinner sections to far higher temperatures than during normal operation.

W. Unruh

Vancouver, Canada

An incandescent lamp produces light by heating a tungsten wire filament to about 2500 °C. At high temperatures tungsten atoms evaporate from the surface of the wire, causing the blackening sometimes seen on the inside of the lamp’s glass envelope. This evaporation also leads to the filament gradually getting thinner with use.

A filament-destroying hotspot can occur on a filament for two reasons. Firstly, if some turns in the tungsten coil are slightly closer than average, the temperature of these compressed turns will be higher than normal because more of the radiation they emit is trapped. Secondly, some turns of the coil may be a little thinner than those on either side of them. These turns will have a higher resistance than normal.

Therefore the rate of heat production at hotspots will be higher than in adjacent sections, and the thinner section will also have a reduced surface area, which decreases the rate at which heat can be lost, again contributing to a higher than normal temperature.

Because the rate of evaporation increases exponentially with temperature increase, hotspots will thin faster than cooler sections. Furthermore, as the wire at the hotspot thins, its resistance increases, raising the temperature even more. So the temperature will continue to rise and wire-thinning will take place at an accelerating rate.

The cold resistance of a filament is about a tenth of that at normal operating temperature. This means that when a lamp is switched on, the initial current is very high compared with the normal running current. If the diameter of the wire at a hotspot has become sufficiently small, the large spike of current at switch-on may melt the wire.

As a small gap forms between the broken ends of the filament, an electrical discharge may cause a spark or arc to form across the gap. This arc may spread to the leads supplying current to the filament. If this happens, the low-resistance arc allows a large surge of current to flow through the lamp, which may in turn cause a fuse to blow or a current-breaker to trip. The arc may be seen as a flash of light within the lamp.

Bill Madill

University of Central England, Birmingham, UK