Iron filings

Iron filings are very small pieces of iron that look like a dark gray powder. They are a common by-product of metalworking. Iron filing are often used in schools to show magnetic field lines. When filings are placed in a magnetic field, they become small magnets. This is because iron is a ferromagnetic material. The south pole of each particle attracts the north poles of its neighbors. In this way filings create chains parallel to the direction of the magnetic field.

Iron filings on a magnet.