Ferromagnetic materials consist of clusters called domains. These domains behave like small permanent magnets with each domain having its own north and south pole. In a macroscopically sized piece of “unmagnetized” piece of ferromagnetic material the domain orientation is random thus canceling out each others magnetism, the effect is that you can not see the magnetic field. For a permanent magnet the domains are generally lined up in the same direction.
The lowest energy state for the ferromagnetic material to be in is a random order. If you apply an external magnetic field you can get the domains to line up in the same direction. One way to do this is to wrap a coil wire around the material and apply an electric current to the coil. because each domains properties vary you may have to increase the current to get all of the domains to point in the same direction. As the number of domains lining up increases the magnetic field will get stronger. At some point all of the domains will line up. Once they are all lined up the adding more current will not increase the strength of the magnetic field.
In ferromagnetic materials a magnetizing field of one oersted can result in a magnetic field of several thousand gauss. This is because the domains are providing the field, they just need a little energy to to move into the desired orientation.
The gauss, abbreviated as G or Gs, is the cgs unit of measurement of a magnetic field B, which is also known as the “magnetic flux density” or the “magnetic induction”. It is named after German mathematician and physicist Carl Friedrich Gauss. One gauss is defined as one maxwell per square centimeter. The cgs system has been formally superseded by the SI system, which uses the tesla (T) as the unit for B. One gauss equals 1×10−4 tesla (100 μT) (1 T=10,000 G).