|I am interested in finding out if and what materials affect magnetic fields. — HLD, Jacksonville, FL
Magnetic fields are associated with lines of magnetic flux, invisible structures that stretch between north and south magnetic poles or that curve around on themselves to form complete loops. Unless a material has its own north or south magnetic poles, it can't terminate the magnetic flux lines and can have only small effects on magnetic fields. The few materials that do affect magnetic fields substantially are ones such as iron or steel that are intrinsically magnetic and that can easily develop strong north and south magnetic poles. These magnetic materials can significantly shift the paths of the magnetic flux lines. If you put an iron or steel box in a magnetic field, the flux lines will tend to travel through the walls of the magnetic box. As a result, there will be few magnetic flux lines inside the box and almost no magnetic field. This effect is used to shield sensitive equipment such as the picture tubes in televisions from magnetic fields.