State qualitatively the effect of inserting an iron core into a current-carrying solenoid

If an iron core is inserted into a current-carrying solenoid, it acts as a magnet and the magnetic field get increased and become very strong. If the current in the solenoid is switched off, the magnetization of the soft iron core disappears.

Extra information for reference only
If a soft iron bar is inserted inside a current-carrying solenoid, then the magnetic field inside the solenoid will increase because it magnetizes the iron bar as it can easily gain magnetic properties when current is passed around it and loses the magnetic properties as the current is switched off.
And, as a result, the strength of the magnetic field inside the solenoid gets increased. 

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