The current carrying wire wound spirally in the form of helix about an axis, as shown in is known as solenoid or coil. Magnetic field produced due to the current carrying solenoid is fairly uniform over a small region in the middle of the coil. It acts just like a bar magnet having north and south poles.

                         Solenoid                                                            Helix Rule

There are several methods used to determine the polarity of the solenoid.

(i)    By use of compass needle : If one of the poles (say north pole) of the compass needle be brought into close proximity to one of the poles of the current carrying solenoid of unknown polarity the action of the compass needle will immediately classify the pole as north or south depending upon whether the needle is repelled or attracted.

(ii)    Helix rule: If the helix is held inn right hand in such a manner that the finger tips point in the direction of flow of current and thumb is out stretched longitudinally along the coil, it will point towards North Pole.

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