41. Directions. (A) Sprinkle some iron filings upon a sheet of paper.
(B) Bring one pole of B M in contact with the filings ([Fig. 9]), and lightly sweep it through them several times, always in the same direction. Are the filings simply pushed about?
(C) Do the same with a stick, and compare the result with that produced with B M.
42. Theory of Magnetism; Magnetic Saturation. This bringing into line the particles of iron indicates that each particle became a magnet. This experiment should aid in understanding what is thought to take place when steel is magnetized. The pile of filings represents the body to be magnetized, and each little filing stands for a particle of that body. A bar of steel is composed of extremely small particles, called molecules. They are very close together and do not move from place to place as easily as the pieces of filings. A magnet, however, when properly rubbed upon the steel, seems to have power to make the molecules point in the same direction. This produces an effect upon the whole bar.
Each molecule of the steel is supposed to be a magnet. When these little magnets pull together, the whole bar becomes a strong magnet. When a magnet is jarred, and the little magnetized molecules are mixed again, they pull in all sorts of directions upon each other. This lessens the attraction for outside bodies.
Steel is said to be saturated, when it contains as much magnetism as possible. A piece of steel becomes slightly longer when magnetized.
It is thought, by many, that there is a current of electricity around each molecule, making a little magnet of it. (See [electro-magnets].)
EXPERIMENT 19. To find whether soft iron will permanently retain magnetism.