Lesson 4 Electromagnetism – We learned that the strength of a magnetic field is proportional to the current in the wire. We can double the current in a wire by doubling the voltage of the source of electricity. We discovered that when we wrap a coil around an iron core the strength of the electromagnetic field is much greater than the same coil without the iron core. This is due to the atoms in the iron lining up to amplify the magnetic effect.