You need:

1 battery
4 cables with crocodile clips at the end
1 microammetermore info
2 coils 

This experiment explains how a transformer works.
There are two circuits, the primary one and the secondary one. The primary circuit consists of a source of electricity (in the video is the battery) and a coil.
The secondary circuit consists of a coil and an instrument that measures electric current (microammeter). The coils are positioned close to one another.
As the primary circuit closes, electricity "flows" through the primary coil and induces (creates) electric current in secondary coil. Therefore the microammeter gives an indication.
If we put a metallic object in the core (centre) of both coils the induced current has a greater intensity.
Observe carefully the indication of microammeter
Disconnect the battery and notice again the indication of microammeter

Would you like to find out why is that?

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Video size: 6,3MB
Duration: 1:19 min
© 2003, Despina M. Garyfallidou Athens-Greece