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Making an Air-Screw
You will need: Stiff drawing paper or thin card, ruler, pencil, scissors, compass, cork, needle.
Warm air always rises. With a constant source of heat, we can use a current of warm air to turn a miniature turbine or "air-screw."
Take the drawing paper or card, set the compass to 2 inches and draw a circle on it. Reset the compass to half an inch and draw a smaller circle inside the larger.
Carefully cut around the larger circle and then rule 16 or 18 lines across the paper disc, in the manner shown in the diagram.
Cut along these radiating lines but stop each cut at the edge of the inner circle.
To create the turbine-wheel you must give each blade a slight twist, each in the same direction. When the blades have been carefully set in this fashion, insert the blunt end of a needle into the end of a cork and balance the "air-screw" on the point of the needle.
Make sure that the little turbine turns easily on its needle mounting. Now place the completed instrument above a source of heat, such as a radiator or even a lighted lamp. As the warm air rises, it will come in contact with the blades of the air-screw and set the wheel spinning. The greater the heat the faster the wheel will spin.
How to Burn Steel
You will need: Candle, tongs or pliers, steel wool.
Oxygen is necessary if we wish to burn anything. There are many ways of proving this. One of the simplest is shown here.
You may not realize that steel can burn, although this is done every day in industry with the aid of an extremely hot flame obtained by mixing pure oxygen and acetylene. We will use a candle flame!
In order to burn steel we need as much oxygen as possible around it. We need the steel in fine shreds so that the air can circulate freely around them.
The steel wool used in the kitchen is ideal for our purpose.
Take a tuft of steel wool and fluff it out. Now hold the wool with the tongs or pliers and lower it to the candle flame. (Be sure the burning embers will fall onto a metal surface, preferably into the candle tray.) You will be surprised to see your experiment turn into a miniature fireworks display.
Make Paper and Cork Dance under Glass
You will need: A small pane of glass, two books, silk handkerchief, tissue paper, cork, glycerin.
It is surprising to see how easily and quickly a charge of static electricity can be induced in a pane of glass by rubbing it briskly with a silk handkerchief.
Secure the ends of the glass between the pages of two books, raising the glass so that it is about inch from the top of the table. Tear some tissue paper into tiny scraps and spread these out below the glass. Rub vigorously on top of the pane with the piece of silk. Within a few seconds the scraps of tissue paper will appear to "dance" in a lively manner as they are attracted by the static charge of electricity that you are producing in the pane of glass.
A variation of this experiment is to replace the tissue paper with tiny pieces of cork (obtained by cutting and chopping an ordinary bottle cork). These can also be made to "perform" once you have induced a static charge in the pane of glass.
It is possible to produce a sufficiently strong charge of static electricity so that the scraps of cork will hang from the underside of the glass like miniature stalactites.
If you really wish to surprise your friends, however, tell them that the pieces of cork are so obedient that you can make them form the initial of your name.
The secret of this trick is to first have your pane of glass suitably prepared. This is done by smearing an outline of glycerin in the shape of your initial on the underside of the glass.
When you now rub the top of the glass with the silk handkerchief, the pieces of cork will be attracted to the underside. Where they come into contact with the glycerin they actually adhere to the glass.
Stop rubbing the glass and the pieces of cork outside the glycerin outline will fall back to the table, leaving your initial clearly "written" in cork chips.
How Water Spurts
You will need: A length of cardboard tubing, plasticine, water, gimlet.
Take a length of cardboard tubing and use a gimlet to bore four small holes equal distances apart along the side of the tube, as shown in Fig. 1.
Use plasticine as a cork at one end of the tube, forcing the plasticine into position so that the joint is water-tight.
The tube is now ready for your experiment—which will be best performed either in the garden or over a bathtub or large bowl.
Take a large jug of water and fill the tube. Immediately, water will begin to spurt from the four holes. You will find that the holes nearest the top of the tube have the weakest jets. It is the lowest hole, bearing the pressure from the full column of water, which will spurt the farthest.
Make Smoke Obey
You will need: A shoe box, two cardboard tubes, razor blades, stub of birthday candle, pencil, scissors, one sheet of paper.
Here is a neat little trick which will help to mystify your non-scientific friends. Perform it on a metal table top.
Find an empty shoe box or similar container that has a lid. Place two small cardboard tubes in position on the lid, as shown in Fig. 1, and draw around them. With the razor blade, cut holes inside the outlines which you have drawn. The two cardboard chimneys should fit tightly into the holes you have made for them. Replace the lid.
Now roll up and twist a small sheet of paper into a match shape, but about twice as large. This is called a paper spill. Light one end and then quickly blow the flame out. Tell your friend to hold the now smoking paper spill to the top of any of the cardboard chimneys. He will probably be disappointed when he obeys you and finds that nothing exceptional happens.
Now remove the lid. Light your little candle and let a few drops of wax fall on a spot in the bottom of the box exactly below one hole. Place the lighted candle on the wax spot so that it stands up and burns immediately under one of the cardboard tubes (Fig. 3) when the lid is replaced.
This time, when a smoking spill of paper is held to the other chimney, as shown in Fig. 4, the smoke will be drawn down into the box and carried up and out through the second tube.
Although this looks quite remarkable when the experiment is successfully performed, the reason is simple. The burning candle quickly consumes all the oxygen in the box and draws in a further supply from the tube farthest away from it, sending the used warm air out through the chimney immediately above it. This is a natural process for a burning flame. The smoke from the paper spill will also be drawn down into the box and will be seen to issue from the other cardboard tube immediately above the candle.
Finding the Center of Gravity
You will need: Card, pencil, compass, scissors, thread.
The story is told that while Sir Isaac Newton was in his garden an apple fell on his head from a tree. The great scientist immediately began to wonder what caused the apple to fall while the sun, moon and other stars remained overhead and (fortunately) showed no tendency to follow the apple.
The theory of gravity for which Newton is now famous proves valuable in many aspects of our modern life. Airplanes have to be so constructed and powered that they can sucessfully resist the force of gravity. Cars and trucks, and especially tall vehicles, have to be made with their centers of gravity low enough to withstand any tendency to topple over when driven around sharp corners.
Designers and engineers have to work out complicated mathematical formulae to discover the center of gravity of the product they are working on. Provided we use small objects, such as pieces of a card of different shapes, we can discover their centers of gravity.
For the first experiment draw a small circle with the aid of compass and pencil. Cut this disc out and note that it will balance perfectly when a needle point is placed on the center mark left by the compass.
Similarly, cut out a small square of card and draw diagonal lines from the corners of the square. The spot where the diagonals cross each other indicates the center of the square. When you place a needle point at this center you find that the square of card will balance perfectly.
The work of finding the center of gravity of an irregularly shaped piece of card becomes somewhat more complicated, however. First suspend the card from one of its corners by a piece of thread tacked to the wall. When it has settled in position take a ruler and continue the line of the thread straight down across the card. Next, suspend the card by another corner. Again, allow the card to become settled and use a ruler to extend the line of the thread across the card.
The center of gravity of the irregularly shaped piece of card occurs where these two diagonals cross each other. Place the card on a needle point at this spot and it will balance perfectly.
Spaces between Molecules
You will need: Two glasses, cotton, water.
Scientists know that even in solid objects there are many minute spaces between the small particles of matter, which they call "molecules." This simple experiment proves there are spaces. What happens?
Start with the two glasses. Fill one with water, the other with absorbent cotton. The photo above shows the two glasses arranged: one filled with water and the other with cotton.
Now slowly pour the water over the cotton, until one glass successfully holds the contents of both glasses. (See photo below.) Of course you cannot see that the molecules of the water and the wool are now filling the minute spaces that were formerly empty, but you have proved it.
You will need: Wire, glass rod, wooden rod, candle.
We speak of certain materials as being good conductors of heat and others as poor conductors. By this, we mean that whereas one material will readily absorb and pass heat along its length, another will resist it and try to confine the heat to its source.
An ordinary candle flame will permit us to perform a few simple experiments to show the relative heat conductability of various types of materials.
First, hold a glass rod to a candle flame. No matter how long you keep it there, the end of the rod which you are holding will remain unaffected by the heat at the other end. This is because glass is an extremely poor conductor of heat.
Be careful when you remove the rod from the flame, however. Glass always looks so deceptively cool—and this will not be the case with the end of the rod which has been held to the flame. In fact, it will be uncomfortably hot. So take care.
Now try the same experiment with a wooden rod. The end of this will char and may possibly flame up after it has been held to the candle for a few seconds. The end which you are holding will remain cool because wood is also a poor conductor of heat.
Finally, take a length of wire and hold one end of this in the candle flame. Be prepared to drop the wire suddenly, however, for within a very short time the wire will have conducted heat from the candle flame to your finger tips to an uncomfortable degree.
This will prove that although glass and wood are poor conductors of heat, metal is a good conductor. Perhaps you can answer the following question now: Why do saucepans and kettles have wooden handles?
The Candle at the Door
You will need: A candle.
Here is a simple test you can make of convection currents at work.
When a room is being heated, the hot air in the room always rises and seeks to escape. Meanwhile, cold air is drawn into the room at a low level to fill the area of low pressure created by the rising warm air.
Allow a room to get thoroughly warm. Then open the door a few inches and hold a lighted candle to the top of the partly-opened door. The direction of the flame will indicate that there is a current of air escaping from the room.
Now hold the candle as low as possible at the door opening. The movement of the flame (plus the cold draft which you will feel) will indicate that there is a current of cold air flowing into the room.
Now experiment with the position of the candle flame about midway between these two extremes of distance. With patience you will find a spot where the flame burns steadily, indicating that there are no drafts at this particular position.
A Trick of Heat
You will need: A quarter (or other coin), handkerchief, wooden skewer or pencil.
Tell your friends that you can hold a piece of burning wood to a handkerchief without scorching or burning the cloth.
(If your mother is not too anxious to see this experiment performed with a good linen handkerchief, let her find you an old piece of linen from her rag bag.)
Place a quarter in the center of the handkerchief and fold the cloth over the coin, twisting the ends together so that the handkerchief is drawn tightly across the face of the coin.
Now place the end of a pencil in the flame of a candle until the wood is glowing red. Press the end of the smoldering wood for 10 seconds against the handkerchief. When the pencil is removed and any loose ash blown away from the cloth there will be no sign of scorching.
This is because the metal coin is such a good conductor of heat that it carries the heat from the end of the smoldering wood right through the handkerchief with such speed that it has no time to scorch the cloth.
The Power in Air Pressure
You will need: Water, a screw-top can, source of heat.
We have seen something of air pressure at work in other experiments in this book. The present experiment shows the quite startling pressure which air can exert under certain circumstances.
You must obtain an empty clean tin can with a well-fitting screw-top that makes it airtight. The larger the can, the more spectacular the experiment will be.
Pour half a cupful of water into the can and then place it (without the screw-top) on a stove and allow the water to boil.
When steam is issuing from the mouth of the can, remove the can from the source of heat and screw the cap tightly into position. (Use a cloth to hold the can.)
As the can cools you will notice that its sides begin to show signs of strain. By the time the can has properly cooled, the sides will be buckled inward to such an extent that you may well be surprised that air pressure alone could have caused the damage.
When the water boiled, the steam and water vapor thus generated forced most of the air out of the can. The screw-top prevented the return of the air and as the can cooled, the steam trapped inside it condensed again into water and thus lowered the pressure inside the can.
We know that air detests vacuums or areas of low pressure and does its best to secure an entry into them. It was the pressure from the outer air as the steam inside the can condensed, which caused the sides of the can to bend inward in such a spectacular manner.
Testing the Skin of Water
You will need: Water, water glass, eye-dropper, tissue paper, razor blade, soup plate.
Of course, water has no real "skin" but it does have a tension at the surface that can be readily demonstrated by experiment. For instance, it is possible to overfill an ordinary glass so that the water stands some one-eighth of an inch higher than the edges of the glass.
Take a dry glass and fill it almost to the top with water, taking care that none spills down the sides at this stage. Place the glass in the soup plate and then use the eye-dropper to add further water to the glass until the level is well above its edges. It is the surface tension of water which allows you to overfill the glass in this fashion. A further proof of this tension can be obtained by floating a razor blade on water. Place the razor blade on a small piece of tissue paper and float the paper on the surface of the water. After a minute or two the paper will become saturated (i.e., all the air will be driven from the paper and replaced by water) and it will sink to the bottom of the glass leaving the razor blade floating on the surface. You can use a needle in place of the razor blade.
Excerpted from Science experiments and amusements for children by Charles Vivian, S.A.R. Watts. Copyright © 1963 Sterling Publishing Co., Inc.. Excerpted by permission of Dover Publications, Inc..
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