1.2.3 Manetic Field due to Circular Conductor

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      Consider an arrangement in which a long conductor is wound with number of turns on a core, close together to form a coil. This is called a solenoid as shown in the Fig. 1.4 

                                               

       When such a conductor carries a current, the magnetic field gets produced around the core.    

       Identifying  the direction of flux and hence identifying the two ends of the core as N pole or S pole is important in understanding the principle of d.c. machine. The right hand thumb rule can be modified for such case as started below.

       The right hand thumb rule : Hold the solenoid in the right hand such that curled  fingers point in the direction of the current though the curled conductor, then the outstretched thumb along the axis of the solenoid points to the North pole of the solenoid or points in the direction of flux lines inside the core.

                                     

                        Whith this background, let us start the detail study of a d.c. generator.

     

      key point : The direction of flux can be reversed either by changing direction of current though the conductor by  reversing the polarities of the supply ore by changing the direction of winding of the conductors around the core.