It is a measure of how superior a specific material is than air as a path for magnetic lines of force. Another characteristic of magnetic material is saturation. It is the maximum value of magnetic induction at a specified field strength. When a material saturates, it losses its linearity.
Magnetic materials are available in many different types and sizes.
These cores are composed of finely defined particles of iron which are insulated from each other but bound together with a binding compound. The iron powder and binding compound are mixed and compressed under heavy pressure, and baked at high temperature. The characteristics of the cores are determined by the size and density of the core, and the property of the iron powder. The high temperature stability makes it suitable for applications such as narrow band filter inductors, tuned transformers, oscillators and tank circuits. See Section I for more details on materials, shapes and sizes.