The last word–

CHARGERS GET SLIMModern cellphone chargers are much lighter and less bulky than they used to be. I assume that this is because they no longer use a transformer with a metal core to step the voltage down. So how do they work?

● The two replies published previously (Last Word, 21 April 2007) on the reduction in size of chargers contained factual inaccuracies. The size of a transformer is only inversely proportional to the frequency of the electricity supplying it if the flux density remains the same.

Second, ferrite is not iron filings glued together; it is a ceramic material where iron is combined with one of several metals – usually manganese or nickel.

In addition, the transformer cores made of iron filings glued together are called “iron powder” and, finally, the transformer in a “switched-mode” supply is rarely torus-shaped.Ian Benton,Ashton-in-Makerfield, Lancashire, UK

STUFF THATI cooked some poultry stuffing and left it in a bowl in the fridge overnight, covered with aluminium foil. In the morning there were holes in the foil where it had touched the stuffing, which was stained black under each hole. Uncooked stuffing does not produce this effect, and it makes no difference whether the stuffing is cooked inside the bird or separately. What

is going on here, and are the black stains poisonous?

● Without its submicroscopic insoluble skin of oxide, aluminium cookware would catch fire easily.

Normally, breaks in the oxide skin of aluminium heal instantly when the exposed metal reacts with say, air or water. But if, for example, mercury or certain alkalis or acids dissolve this skin, the exposed underlying metal reacts vigorously. So, while aluminium cookware and foil are safe and useful in the kitchen, it is important to keep them away from strong salt solutions or caustic soda, for example, and also from wet food when it is not actually cooking.

Wet, fatty materials such as cooked lard form fat-soluble detergents that penetrate microscopic chinks in the oxide layer , exclude air that otherwise

would reseal the skin, and corrode pinholes into the metal. If floating fat has coated the metal, even cold chicken soup can eat through a thick aluminium pot overnight.

The black stain is mainly from small amounts of iron in the aluminium. It is not deadly,

but it is better not to eat food contaminated with high levels of metals, which also spoil the taste. For wrapping cooked fatty or acidic food for more than short periods, plastic film is much better.Jon RichfieldSomerset West, South Africa

THIS WEEK’S QUESTIONSFilthy lucre?Can viruses and bacteria be transmitted on coins or notes, which pass through so many hands? What is the likelihood of catching something unpleasant from money, and if it does harbour disease, what might I catch? Finally, what is the average number of germs likely to be hitching a ride on coins or paper money?Michaela LanzarottiPesaro, Italy

Highly sprungHere are six springs taken from various ballpoint pens (see Photo, left). Why are they so different from each other? In particular, why do some pens have a “squashed-up” bit in the middle? And is it unusual for someone to dismantle pens compulsively during meetings?Hugh HuntCambridge, UK

Down in oneWhy is it so much easier to drink a whole pint of beer or orange squash, say, in one go than it is to down a pint of water?Geoff LaneBury, Lancashire, UK

Sick as a horseOn a long motorway journey while driving behind a horsebox, I wondered, do horses get travel sick? In fact, do we know whether any animals besides humans suffer from motion sickness?Neil BowleyNewthorpe, Nottinghamshire, UK

Last Words past and present, plus a full list of unanswered questions, are available at www.newscientist.com/lastword

“If floating fat has coated the

metal, even cold chicken soup

can eat through a thick

aluminium pot overnight”

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