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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.
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