<img src="https://d10lpgp6xz60nq.cloudfront.net/physics_images/CPC_SPP_SCI_X_C13_E02_014_S01.png" width="80%"> <br> Principle : A current - carrying conductor, when placed in a magnetic field. Experiences a force. If the direction of the field and that of the current are mutually perpendicular to both and will be given by Fleming.s left hand rule due to this force the conductor begins to move. <br> Working : Current in the coil ABCD enters from the source battery through conducting brush X and flow back to the battery through brush Y, The current in arm AB of the coil flows from A to B, In arm CD it flows C to D, that is opposite to the direction of current through arm AB We find that force acting on arm AB pushes it downwards while the force acting on arm CD pushes it upwards Thus the coil and the axle O, mounted free to turn about an axis, rotate anti-clockwise, At half rotation, Q makes contact with the brush X and P with brush Y. Therefore the current flows along the path DCBA, The reversal of current also reverses the direction of force acting on the two arms AB and CD. Thus the arms AB of the coil that was earlier pushed down is now pushed up and the arm CD previously pushed up in now pushed down, Therefore the coil and the axle rotate half a turn more in the same direction. The reversing of the current is repeated in each half rotation giving rise to a continuous rotation of the coil and the axle. <br> The split rings help to reverse the direction of current in the circuit, there are called the commutator.