Electric current is "induced" (produced) in the secondary circuit only the intensity of the electric current in the primary circuit changes, that means while the primary circuit opens or closes. The phenomenon of induction does not appear when in the primary circuit there is no electric current or when the intensity (amperage) is still. 
Therefore the change in the intensity of the electric current, generates a magnetic field with changing intensity, which in turn generates electric current again.
The operation of transformers is based on the phenomenon of the induction. Transformers are used in many devices such as chargers for mobile phones and portable computers. Large transformers used by the PPC.

parts of transformer
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© 2003, Despina M. Garyfallidou Athens-Greece