In the figure given below, a coil is placed between two permanent magnets and is rotated with the help of an axle on which two rings R1 and R2 are internally attached. B1 and B2 represents two metallic brushes. G represents galvanometer.
In the given setup, if the axle moves with constant speed the current through the arm AB and CD :
reverses direction periodically.
If the coil is rotated in clockwise direction, the current flows as shown in the figure (Fleming's right hand rule), after half a period i.e. when the coil flips over once, current in AB moves in downward direction.
Similar reversal of current takes place in CD as well. Thus the current in the circuit reverses direction.
As AB moves over again to its initial position after one complete rotation the current would again reverse direction.
Hence the current reverses direction and this happens periodically.