Magnetic field of wire around a nail. Iron filings on a sheet of paper after the removal of a nail and the wire wrapped around it. The wire wrapped around the nail had electric current flowing in it that magnetized the nail (the wire was acting as a solenoid or electromagnet). The filings mark the lines of the resulting magnetic field. The lines are concentrated where the field was strongest, in this case inside the actual nail. This is shown by the horizontal line ending in the black spot at left (the head of the nail). The other, looping lines, radiate from pole to pole of the resulting magnetic system (similar to a bar magnet).