Visualizing Magnetic Fields

This video is about visualising magnetic fields of permanent magnets and coils using (a) an array of compass needles and (b) iron filings.
A compass needle is a small, linear magnet that can rotate freely. When the compass is far away from other magnets or objects made of ferromagnetic materials such as iron, it aligns (approximately) along the north-south direction. The end of the compass pointing in the general direction of geographic north is called (magnetic) north pole. However, if a stronger magnet is brought near the compass needle, the needle will align tangent to the direction of the magnetic field lines.
Fields lines are imaginary lines that allow us to visualize the magnetic field around a magnet, electromagnet or any current-carrying conductor.
A small compass needle will always align along such a field line.
Iron filing are small pieces of iron that, in the presence of a magnetic field will align along the field lines and thus allowing us to visualize magnetic fields.
As magnetic field is a vector, it has a direction.
For permanent magnets, the direction of the magnetic field lines is: out of the north pole and into the south pole.
For electromagnet, special rules need to be used in order to determine the direction of the field lines (e.g. cork screw rule or right-hand grip rule)
The vector magnetic field (B) is always tangent to a field line and its direction coincides with that of the field line.
Note that, although not directly obvious in this video, the magnetic field lines are closed loops. Also, the magnetic field lines do not intersect.