Fig. 82.—Showing how the Lines of Force "Leak" at the sides of the coil, from a Coil of Wire, and how they are concentrated by an Iron Core.
The presence of the iron bar inside of the coil of wire greatly increases the number of lines of force running through the coil.
Fig. 83.—The Principle of an Electro-Magnet.
When a bar is not used, many of the lines of force leak out at the sides of the coil, and but few extend from end to end. The effect of the iron core is not only to diminish the leakage of the lines of force, but also to add many more to those previously existing. Hence the magnetic strength of a coil is greatly increased by the iron core.
A coil of wire wrapped around an iron core forms an electro-magnet.
Fig. 84.—if you wrap some insulated Wire around an Ordinary Nail and connect it to a Battery, it will become an Electro-Magnet.
If you wrap some insulated wire around an ordinary nail and connect it to one or two cells of a battery it will become an electro-magnet and pick up bits of iron and steel.