The magic ruler

What you’ll need

  • plastic ruler
  • an item of clothing made of wool (not cotton), e.g., socks or a pullover
  • for the first trick: a sink with a faucet
  • for the second trick: an empty aluminum can

How to do it

First trick: 1. Turn the faucet on just a little to get a very thin but continuous stream of water. It doesn’t matter whether the water is hot or cold.

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2. Rub the ruler against the wool. Hold it close to the stream of water. What happens?

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Second trick: 1. Place the aluminum can on the table and rub the ruler against the wool.

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2. Now place the ruler near the aluminum can. What happens?

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Well spotted!

The water "bends" towards the ruler. The aluminum can is drawn towards the ruler and rolls after it.

What's the secret?

The smallest particles in the water – called molecules – contain negatively charged electrons and positively charged protons. They are arranged in such a way that each water molecule has one positive and one negative pole. When a water molecule comes near an electrically charged object, it aligns itself like a small magnet in the electric field.

When you rubbed the ruler against the wool, electrons jumped from the fabric onto the ruler, so that the ruler has more electrons than protons and is negatively charged.* If you place the ruler near the water, it attracts the positive pole of the water molecules and "bends" the water.

Aluminum is a conductive material. When the negatively charged ruler is moved near the aluminum, the electrons in the aluminum flow to the other side of the can furthest from the ruler. The side facing the ruler is then positively charged and is drawn towards the ruler.