ClipArt ETC
"Illustrating the effects of introducing an iron core into a solenoid. In the upper firgure, the air space or 'air core' surrounded by the solenoid offers considerable resistance to the passage of magnetic lines, allowing only a small number to pass through. If a pieve of iron be introduced, as in the lower figure, the number of lines will be greatly increased." -Hawkins, 1917

Effect of a metal core in a solenoid

"Illustrating the effects of introducing an iron core into a solenoid. In the upper firgure, the air…