When iron is placed in a solenoid with current flowing through the solenoid circuit, the iron becomes magnetized, adding the lines of its own magnetic flux to the magnetic lines produced by the current. The total flux per square centimeter is no longer numerically equal to the magnetizing force, but to a larger quantity called the magnetic induction.
This quantity is represented by the letter B, where B is the sum of the magnetic lines produced by the current and those produced by the iron.