Scenting coins

Three different coins lie in a plastic dish. You close your eyes while another person takes out one coin, holds it for several seconds in his closed hand, and puts it back. 

Now hold the coins one after the other briefly to your upper lip and find out immediately, to everyone’s astonishment, which coin was taken from thedish.Since metal sare very good conductor sof heat,the coin warm sup immediately in the hand. 

But plastic is a poor conductor, so hardly any heat is lost to the dish when the coin is put back. The upper lip is particularly sensitive and reveals the smallest temperature difference in the coins, so that you can detect the right one immediately. Before the trick is repeated it is a good idea to lay the coins on a cold stone floor to conduct away the heat.

Fire Guard

Hold a metal kitchen sieve in a candle flame. To your surprise the flame only reaches the wire net, but does not go through it.

The metal in the sieve conducts so much heat away that the candle wax vapour cannot ignite above the wire net. The flame only passes through the metal lattice if it is made to glow by strong heating.

Theminer’s safety lamp works in the same way.A metall attice surrounding the naked flame takes up so much heat that the gases in the mine cannot ignite.

Non inflammable material

Place a coin under a cotton handkerchief and ask someone to press a burning cigarette on the cloth stretched over the coin.

You need not be afraid of scorching the material, because only a harmless speck of ash will be left.

The experiment shows that the metal of the coin is a much better conductor of heat than the cotton fabric. 

On rapid pressure the heat of the burning cigarette is immediately conducted away by the coin. There is only enough heat to cause a small rise in temperature in the coin, and the cotton does not reach a high enough temperature to burn.

A clear case

Put spoons made of steel, silver, and plastic and a glass rod into a glass. Fix a dry pea at the same height on each handle with a dab of butter. 

In which order will the peas fall if you pour boiling water into the glass?

The butter on the silver spoon melts very quickly and releases its pea first. 

The peas from the steel spoon and the glass rod fall later, while that on the plastic spoon does not move. Silver is by far the best conductor of heat, while plastic is a very poor conductor, which is why saucepans, for example, often have plastic handles.

Expanding Metal

Take an empty, corked wine bottle, push as long an aluminium knitting needle as you can find into the bottle cork and let the other end project under slight pressure over the mouth of a second, uncorked bottle. 

Glue a paper arrow on to a sewing needle, making sure that it is balanced, and fix it between the knitting needle and the neck of the bottle. Place a candle so that the tip of the flame touches the middle of the needle and watch the arrow.

The arrow turns quite quickly some way to the right because the knitting needle expands on heating like other substances. With an ordinary steel knitting needle the arrow would only turn a little, because steel only expands half as much as aluminium. 

Since the aluminium is longer as well, the difference is still greater.The expansion is clearly visible in electricity power cables, which sag more in summer than in winter. If you take the candle away from the knitting needle, the arrow moves back.

Hot Air Balloon

Roll a paper napkin into a tube and twist up the top. Stand it upright and light the tip. While the lower part is still burning, the ash formed rises into the air. 

Take care! The air enclosed by the paper is heated by the flame and expands. The light balloon-like ash residue experiences surprising buoyancy because the hot air can escape, and the air remaining in the balloon becomes correspondingly lighter.

Veryfinenapkinsarenotsuitablefortheexperiment because the ash formed is not firm enough.

Buddel Thermometer

Pour some coloured water into a bottle. Push a drinking straw through a hole bored in the cork so that it dips into the water. Seal the cork with glue. 

If you place your hands firmly on the bottle, the water rises up the straw.The air enclosed in the bottle expands on heating and presses on the water surface. 

The displaced water escapes into the straw and shows the degree of heating by its position. You can fix a scale on the side of the bottle.

Expanded air

Pull a balloon over the mouth of a bottle and place in a saucepan of cold water. If you heat the water on a stove, the balloon is seen to fill with air.

The air particles in the bottle whirl around in all directions, thus moving further apart, and the air expands. This causes an increased pressure, which escapes into the balloon and causes it to distend. If you take the bottle out of the saucepan, the air gradually cools down again and the balloon collapses.

Bottle Ghost

An empty wine bottle, which has been stored in a cool place, has a ghost in it! Moisten the rim of the mouth with water and cover it with a coin. Place your hands on the bottle. Suddenly the coin will move as if by a ghostly hand.

The cold air in the bottle is warmed by your hands and expands, but is prevented from escaping by the water between the bottle rim and the coin. 

However, when the pressure is great enough, the coin behaves like a valve, lifting up and allowing the warm air to escape.

Coin in the well

Place a coin in a dish of water. How can you get it out, without putting your hand in the water or pouring the water from the dish? 

Put a burning piece of paper in a tumbler and invert it on the dish next to the coin. The water rises into the tumbler and releases the coin.

During combustion the carbon contained in the paper, together with other substances, combines with the oxygen in the air to form carbon dioxide. 

The gas pressure in the tumbler is reduced by the expansion of the gases on heating and contraction on cooling. The air flowing in from outside pushes the water into the tumbler.

Twin Tumblers

Light a candle stump in an empty tumbler, lay a sheet of damp blotting paper over the top and invert a second tumbler of the same size over it. After several seconds the flame goes out and the tumblers stick together.

During combustion the oxygen in both tumblers is used up - the blotting paper is permeable to air. Therefore the pressure inside is reduced and the air pressure outside pushes the tumblers together.

Explosion in a Bottle

Throw a burning piece of paper into an empty milk bottle and stretch a piece of balloon rubber firmly over the mouth. After a few moments, the rubber is sucked into the neck of the bottle and the flame goes out.

During combustion, part of the expanded, hot air escapes.

After the flame goes out the diluted gas in the bottle cools and is compressed by the external pressure. The rubber is therefore stretched so much that the final pressure equalisation only occurs if you break the bubble, causing a loud pop.

Wind Funnel

Light a candle and blow at it hard through a funnel held with its mouth a little way from the flame. You cannot blow out the flame; on the contrary it moves towards the funnel.

When you blow through the funnel the air pressure inside is reduced, and so the air outside enters the space through the mouth. 

The blow air sweeps along the funnel walls: if you hold the funnel with the edge directly in front of the flame, it goes out. If you blow the candle through the mouth of the funnel, the air is compressed in the narrow spout, and extinguishes the flame immediately on exit.

Floating Card

Many physical experiments seem like magic, but there are logical explanations and laws for all the strange occurrences. Stick a thumbtack through the middle of a halved postcard. 

Hold it under a cotton spool so that the pin projects into the hole and blow hard down the hole. If you manage to loosen the card, you really expect into fall. In fact, it remains hovering under the spool.

Bernoulli’s law explains this surprising result. The air current goes through at high speed between the card and the spool, producing a lower pressure, and the normal air pressure pushes the card from below against the spool. The ascent of an aeroplane takes place in a similar manner. 

The air flows over the arched upper surface of the wings faster than over the flat under-surface, and therefore the air pressure above the wings is reduced.

Flying Coin

Lay a sixpence or a dime four inches from the edge of the table and place a shallow dish eight: inches beyond it. How can you blow the coin into the dish!

You will never do it if you blow at the coin from the front - on the false assumption that the air will be blown under the coin because of the unevenness of the table and lift it up. It will only be transferred to the dish if you blow once sharply about two inches horizontally above it. 

The air pressure above the coin is reduced, the surrounding air, which is at normal pressure, flows in from all directions and lifts the coin. It goes into the air current and spins into the dish.

Trapped Ball

Place a table tennis ball in a funnel, hold it with the mouth sloping upwards, and blow as hard as you can through the spout. 

You would hardly believe it, but nobody can manage to blow the ball out.The air current does not hit the ball, as one would assume, with its full force. 

It separates and pushes through the places where the ball rests on the funnel. At these points the air pressure is lowered according to Bernoulli’s law, and the external air pressure pushes the ball firmly into the mouth of the funnel.

Wind-proof coin

Push three pins into the middle of a piece of wood and lay a coin (5 new pence or 25-cents) on top of them. You can make a bet! Nobody who does not know the experiment will be able to blow the coin off the tripod.

The metal cannot hold the gust of air on its narrow, smooth edges. The gust shoots through under the coin

and reduces the air pressure, forcing the coin more firmly on to the pins. 

But if you lay your chin on the wood just in front of the coin and blow with your lower lip pushed forward, the air hits the underside of the coin directly and lifts it off.

Bernoulli was right

Lay a postcard bent lengthways on the table. You would certainly think that it would be easy to overturn the card if you blew hard underneath it. 

Try it! However hard you blow, the card will not rise from the table. On the contrary, it clings more firmly.

 Daniel Bernoulli, a Swiss scientist of the eighteenth century, discovered that the pressure of a gas is lower at higher speed. 

The air stream produces a lower pressure under the card, so that the normal air pressure above presses the card on to the table.

Curious Air Currents

If you stand behind a tree trunk or a round pillar on a windy day, you will notice that if offers no protection, and a lighted match will be extinguished.

A small experiment at home will confirm this: blow hard against a bottle which has a burning candle standing behind it, and the flame goes out at once. 

The air current divides on hitting the bottle, clings to the sides, and joins up again behind the bottle with its strength hardly reduced. It forms an eddy which hits the flame. You can put out a lighted candle placed behind two bottles in this way, if you have a good blow.

Egg Blowing

Place two porcelain egg-cups one in front of the other, with an egg in the front one. Blow hard from above on to the edge of the filled cup. 

Suddenly the egg rises, turns upside down and falls into the empty cup. Because the egg shell is rough, it does nor lie flat against the smooth wall of the egg-cup. 

Air is blown through the gap into the space under the egg, where it becomes compressed. When the pressure of the cushion is great enough, it lifts the egg upwards.