A repulsive device

25th November 2005 at 00:00
Invented by the French physicist Jean Antoine Nollett more than 200 years ago, the electroscope is an instrument that measures static electric charges.

To make an electroscope, take a tall drinking glass and measure its height. Cut out a circle of cardboard that will fit snugly on the top and make a hole in the centre of the circle.

Take a piece of copper wire that is thick enough to hold its shape when you hold it upright. The length of the wire needs to be about three quarters the height of the glass.

Push the end of the wire through the cardboard lid, until about 4cm of the wire has come through. Secure it in place. Bend the other end of the wire to make a right angle, ensuring that the bend is about 1cm from the end of the wire.

Take a small piece of foil from a sweet wrapper. This foil needs to be very thin and will form the leaves of the electroscope. Flatten the foil wrapper and fold it in half lengthways. Place it carefully over the bent end of the wire. Lower the wire gently into the glass until the cardboard lid meets the top of the glass. Check that the foil wrapper is still in place, and secure the cardboard lid with tape.

Take a piece of aluminium foil 15cm square and roll it into a ball. Push the ball of aluminium foil onto the end of the wire.

Your electroscope is now ready. The electroscope is able to measure static charges, but does not differentiate between positive and negative charges.

To test your electroscope, charge up a balloon by rubbing it on your jumper. Hold the balloon near to the aluminium foil ball. The foil wrapper should open out. The greater the charge on the balloon the further apart the two ends of the wrapper will move.

A positively charged object will induce negative charges in the foil ball.

This will cause a positive charge on the leaves of the electroscope. Like charges repel, and as each of the leaves of the electroscope has a similar charge, they repel each other.

Sophie Duncan is project manager for science at the BBC www.bbc.co.uksn

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