A magnetic circuit is considered as the path in space through which magnetic flux passes. The figure below shows an iron-cored solenoid. When we supply DC through the solenoid, it will produce flux that's pattern is shown in that figure. Each flux line, as considered, initiated from N pole, passing through the air surrounding the magnet and finally reached to S pole and then from S pole it will come to the N pole through the iron core as shown. As each of the flux lines pass through the air as well as iron, this is called composite magnetic circuit. The lines of force inside the iron core are represented by numbers of uniformly spaced, paralleled lines.
As a result the magnetic field within the iron core is uniform. These lines of force or flux lines through the airspace, are not equally placed at all points, hence, field outside the core is not uniform. In order to make the design and analysis of a magnetic circuit easier, it is desired to produce a uniform field. Instead of using a straight iron core, if we have used an uniform cross-sectioned iron toroid, ideally, there would not be any scope for magnetic flux lines to pass through air. As a result the magnetic field within the toroid core is uniform. This is referred as completely enclosed magnetic circuit.