Why does a piece of iron in a current-carrying coil, or solenoid, increase the magnetic field strength? The iron acts as a capacitor and stores spinning electrons, thus increasing the magnetic field. The magnetic field of the solenoid is amplified because it magnetizes the iron; the magnetic field of the iron bar adds to the magnetic field of the solenoid. The iron is a permanent magnet and adds to the field of the current-carrying loop. The presence of the iron decreases the resistance in the current-carrying loop, thereby allowing more current to flow