Canned heat
When my father was undergoing military service in the desert, his medical officer told him that should he ever become separated from his unit and stranded in the dunes the best course of action would be to drink his water supply 鈥� a 2-pint (1.1-litre) flask 鈥� over the first couple of hours rather than trying to eke it out for days. Is this true? Would either course of action be more likely to help him survive? And why?
鈥� An old US army training film described a soldier鈥檚 water supply as the total in his canteen and in his body, suggesting he should keep the water inside himself instead of outside. It featured a cartoon of a foolish soldier who refused to drink his ration, and got so thirsty and dehydrated that he became irrational. He went out in the midday sun looking for water. By the time he drank from his canteen, the reservoir in his body had all evaporated in sweat, and he soon perished. A wiser soldier topped up his internal reservoir from his canteen, kept his mind in order, stayed in the shade until evening, and hiked to safety.
That said, if you drink up all your supply at the start you might urinate a lot of it right back out. You can help your body to conserve water by not drinking until you are fairly thirsty. Your urine will be scanty and darker than normal 鈥� which is acceptable in such an emergency, though unhealthy in the long run. But at all times you must drink enough to stay mentally alert and avoid heat exhaustion.
Advertisement
Spencer Weart
American Institute of Physics
College Park, Maryland, US
鈥� The advice to drink the 2 pints of water in the first few hours rather than eke it out is sound for a number of reasons. First, dehydration is rapidly accompanied by irritability and mental confusion, which is then followed by physical dysfunction. Such symptoms do not make for good decision making nor efficient action.
Trials on military personnel in Australia have shown that maintaining adequate hydration is essential for efficient mental and physical activity and gives you a far greater chance of survival than trying to stretch out a limited water supply.
The actual survival time differs little, as the graphs of performance show that those who maintain adequate hydration can operate very efficiently until the water runs out, at which point they begin to deteriorate rapidly. Those who eke out their supplies decline almost immediately although at a slower rate. As gruesome confirmation of this, several people have been found dead of dehydration while still carrying water.
Secondly, if you take small sips of water, much of it merely moistens the membranes of the mouth and evaporates. You should swallow at least a cup of water at each drink, so most of it reaches the stomach and is absorbed.
And if you do become lost with only limited water, it is important not to eat because digestion uses up water. You can live and function reasonably well for weeks without food, but in hot conditions one or two days without water can kill.
Eric Wheatley
Bridgetown, Western Australia
Bright spark
If I accidentally leave a metal object in my microwave, such as a spoon in a coffee cup, sparks fly when the oven is switched on. Yet the oven I have just bought came with a metal stand, which doesn鈥檛 spark at all. Why?
鈥� Only very thin sheets of metal or those with sharp edges cause sparks to fly in a microwave oven. Aluminium foil is a good example.
Because we are discouraged from placing metal objects inside microwave ovens you may be surprised to learn that the inside surface is coated with metal. This is because metals behave like a mirror and reflect microwaves.
As well as acting as a Faraday cage by preventing the microwaves leaking out and cooking the chef, the metal coating reflects the microwaves back towards the food so it cooks more efficiently 鈥� unless shielded by foil. Food wrapped in foil will escape cooking and may also lead to some interesting pyrotechnics.
Like all electromagnetic waves, microwaves are composed of alternating electric and magnetic fields. This causes the electrons in metals to move around.
Because it is thin, metal foil has a high resistance. This means that it will heat up, and even ignite, when a current flows in it, in response to microwaves. The movement of electrons only takes place in a thin layer a fraction of a millimetre thick on the surface of the metal 鈥� the so-called 鈥渟kin effect鈥�. So you might imagine that this heating would be a problem with any metal object.
However, while metallic decorations on fine porcelain tend to heat up enough to do damage, in a thicker piece of metal like a spoon any heat would be conducted away into the cooler interior. If the spoon has a sharp edge or a point, the charge may accumulate there, creating sparks at a lower potential.
Mike Follows
Willenhall, West Midlands, UK
This week鈥檚 questions
Free the atoms
Oxygen has a slightly greater density than nitrogen. Why don鈥檛 these main constituents of air separate out?
Gerald Leach
London, UK
Powder your boat
Last summer some of my family were part of a crew sailing at night from Cowes on the Isle of Wight to Plymouth on the south coast of England when they ran into a severe and prolonged thunderstorm in Lyme Bay, west of Portland Bill. They noticed that their compass needle was wandering aimlessly and they had to set their course by the faint glow from lights on the shore.
At the same time, they were puzzled by a black gritty powder landing on the boat, even though the nearest cliffs, some miles away, are limestone and there are no obvious sources of black powder. Another relative who used to sail out of Poole harbour in Dorset reported a similar situation off St Albans Head, which is east of Portland Bill. Can anyone offer an explanation?
David Stevenson
Newbury, Berkshire, UK