A dynamo is a device which converts the mechanical energy to electrical energy. The dynamo works on the principle of electromagnetic induction. That is whenever there is a relative motion between a conductor and a magnetic field an emf is induced in the conductor. in the cycle dynamo due to motion of weel, a conductor moves in between poles of a magnet to induce emf in it. And when this conductor is connected to a bulb and the circuit is complete (that is closed) a current flows and bulb glow.