1.3 Revision of Electromagnetism

      When a conductor carries a current, it creates a magnetic field around it. The direction of such magnetic field depends on the direction of the current passing through the conductor.

      So electric current and magnetism are very closely related to each other. This relatoinship plays an important role in the d.c. machines.

      Let us see in brief, the rule to determine the direction of the flux prouduced by a current carrying conductor.
1.3.1 Right Hand Thumb Rule
      It states that "Hold the current carrying conductor in the right hand such that the thumb is pointing in the direction of current and parallel to the conductor, then curled fingers point in the direction of the magnetic field or flux around it".
The Fig. 1.2 explains the rule.
                    

      Conventionally, such conductors are observed, assuming them to be placed perpendicular to the plane of the paper. So current moving away from the observer is denoted by a 'cross' while current coming towards the observer is dentoted in the direction of current denoted as 'cross' i.e. going into the paper, then curled fingers indicate the  
          
      

direction of flux as clockwise, as show in the Fig. 1.3 (a). While if thumb of right hand is adjusted in the direction of the current shown as 'dot' i.e. coming out of paper, then curled fingers indicate the direction of flux as anticlockwise as shown in the Fig. 1.3 (b).