How to change the magnetic field of a coil with a current, having at its disposal an iron rod, the diameter of which

How to change the magnetic field of a coil with a current, having at its disposal an iron rod, the diameter of which is slightly less than the diameter of its hole? How will it change?

The magnetic field of the current coil can be changed by inserting an iron rod into the hole in the coil. In this case, the magnetic field will increase.

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