The Rotating Paper Cylinder

Category: Science
Thu, 18 Oct 2007, 20:51

I first tried this more than three decades ago after reading about it in Martin Gardner's monthly column in Scientific American. It was a fun trick, and at the time, it almost convinced me that I had psychic powers.

To do this "experiment", make a cylinder out of a piece of paper as shown in the following photo. Before you glue the ends together, you may decorate the cylinder as you please. Support the cylinder with another narrow piece of paper pushed through slits on opposite sides of the cylinder, and balance it on a suitable narrow support with a pin or needle. The cylinder should rotate freely. If necessary, adjust the cylinder so that it rests more or less vertically.


With the cylinder balanced and still, carefully cup your hands around the cylinder without touching it, as shown by the following photo:


Concentrate on the cylinder. After a few seconds, the cylinder should start to rotate. Before I offer the explanation for this behavior, think about why this happens.

One possible explanation is telekinesis, the ability to initiate movement without perceptible mechanical means, merely using the power of ones mind. Think of ways to test this hypothesis. Is it even possible to test this hypothesis? Note that if the theory is not testable, it is not, by definition, a scientific theory.

The real reason for the rotation of the cylinder is quite mundane. The cylinder is quite vulnerable to the slightest air currents. By cupping your hands around the cylinder, you are magnifying the slightest currents, increasing the chances that the cylinder will be affected by these currents. Your own breath may itself contribute to the "breeze" surrounding the cylinder.

A clever sort of person, though, could easily convince gullible spectators that his psychic powers moved the cylinder.

I'll leave you with a chemistry joke I heard recently at the Ontario Science Centre. Two atoms are chatting:

First atom: "I just lost an electron."

Second atom: "Are you sure?"

First atom: "Yes, I'm positive!"


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the four elements

The Bottle Diver

Category: Science
Wed, 03 Oct 2007, 12:00

Here's a fun and easy science demonstration you can use to amuse your kids that needs just a two liter plastic pop bottle and a medicine dropper.

First, fill a tall glass with water and drop the empty medicine dropper into it. The eye dropper should float in the water. Next, completely fill the medicine dropper with water. It should now sink in the water. Now, carefully squeeze water out of the dropper until it just floats in the water. Then, squeeze out a couple more drops out of the dropper.

Now, fill the two liter pop bottle with water right up to the top and put the medicine dropper into it. A bit of water should spill out the top of the bottle. Put the cap back on the bottle and tighten it closed. You're now ready for the demonstration.

At first, the medicine dropper should float at the top of the bottle. But when you squeeze the bottle, the medicine dropper should sink. With the right amount of force, you can adjust the depth of the "diver" within the bottle.

Before reading on, try to reason out why this works.

Diver Diver

Ready for the answer? When adjusting the amount of water in the medicine dropper, think about when it sank or floated. With a lot of water, the dropper sank, but with little water, it floated. To put it differently, with little air in the bulb, it sank. Containing a lot of air, it was buoyant and floated.

What happens when you squeeze the bottle? Do you compress the water? Or do you compress the air in the medicine dropper? With a smaller volume of air in the dropper, it becomes less buoyant and sinks.

With this story, I open a new section of my blog devoted to science.


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the four elements