A coil of wire produces a magnetic field when it is energized. The strength of this field depends on the current flowing through it and the distance from it.
The strength of a coils magnetic field can be increased by placing a core of soft ferromagnetic material in the centre of the coil. This can increase the field to thousands of times the strength of the coil alone. The permeability of the core material is also a factor in determining its field strength.
If the core is made from non-magnetic materials such as wood, the permeability will be low. If the core is made from a ferromagnetic material such as iron, steel or nickel alloys, then the magnetic permeability of this material will be much higher. This means that the magnetised flux around the coil will be a lot more concentrated and dense than in a free space such as air.
This is called the “Relative Permeability”. The relationship between relative permeability and field strength is very simple: For example, for an iron core a permeability of 500 is equivalent to a 0.628×10-3 H/m magnetic field.
As the magnetic permeability increases with the size of the core, the field will become saturated. This is a problem for large electromagnets that use a variable current power supply to energize or de-energize the coil.
In the next tutorial, we will look at B-H curves and hysteresis. These are important factors in understanding how the strength of a coils magnetic field can vary with the number of turns of wire within the coil. This will be of interest to anyone who is using a copper coil for electrical applications.