as the basalt cools and solidifies. Geological studies of basalt containing such pieces of magnetised region have provided evidence for the change of direction of earth’s magnetic field, several times in the past. B = µ nI ( . ) If the interior of the solenoid is filled with a material with non-zero magnetisation, the field inside the solenoid will be greater than B .
The net B field in the interior of the solenoid may be expressed as B = B + B m ( . ) where B m is the field contributed by the material core. It turns out that this additional field B m is proportional to the magnetisation M of the material and is expressed as B m = µ M ( . ) where µ is the same constant (permeability of vacuum) that appears in Biot-Savart’s law.
It is convenient to introduce another vector field H , called the magnetic intensity , which is defined by – µ = B H M ( . ) where H has the same dimensions as M and is measured in units of A m – . Thus, the total magnetic field B is written as B = µ ( H + M ) ( . ) We repeat our defining procedure.
We have partitioned the contribution to the total magnetic field inside the sample into two parts: one , due to external factors such as the current in the solenoid. This is represented by H . The other is due to the specific nature of the magnetic material, namely M . The latter quantity can be influenced by external factors.
This influence is mathematically expressed as χ M H ( . ) where χ , a dimensionless quantity, is appropriately called the magnetic susceptibility . It is a measure of how a magnetic material responds to an external field. Table .
lists χ for some elements. It is small and positive for materials, which are called paramagnetic . It is small and negative for materials, which are termed diamagnetic . In the latter case M and H are opposite in direction.