Indian rope trick (magnetism)





This is a good demonstration that is used in schools to teach the idea
of magnetic materials. It can be fiddly to set up, but it's worth the effort.

Materials

   * Clamp stand
   * Magnet
   * Paper clip
   * Cotton
   * Sticky tape
   * Samples of materials to insert in the gap

Method

  1. Clamp the magnet in the clamp stand.
  2. Tie some cotton onto a steel paperclip and allow the paperclip to stick
     to the magnet.
  3. Gently pull the cotton down so that there is an air space between the
     paperclip and the magnet, and then stick it to the bench with some
     sticky tape. This demonstration works best if the air gap is
     sufficiently large that the paperclip is only just held up by the
     magnet. It takes a fair amount of adjustment to get it perfect.
  4. Insert a non-ferromagnetic material into the air gap, e.g. a piece of
     paper or a sheet of copper. The paperclip stays up.
  5. Insert a ferromagnetic material such as an iron nail or sheet of nickel
     and the paperclip falls.

How does it work?

This is a first attempt at an explanation. I've tried to pitch it at a level
accessible to someone with very little knowledge of magnetism. Please
improve it!

The force that holds the paperclip up is strongest near that magnet. If you
have ever sprinkled iron filings on paper over a magnet then you will have
seen the filings line up with one another to form lines that seem to come
out of one end of the magnet and loop around to the other end. These lines
are called magnetic field lines. They represent the magnetic flux. The
closer the lines are together, the bigger the flux and the stronger the force.

Magnetic field lines pass straight through most materials, as if the
materials were not there. Put a piece of paper in the air gap and the
magnetic field lines pass straight through and hold up the paperclip. Some
materials, however, affect a magnetic field. It is as if the lines are
attracted to these materials. If you put something made of these materials
(known as ferromagnetic materials) into the gap, the field lines distort.

If the paperclip is set up so that it is on the brink of falling, then any
disturbance in the field that is holding it up will cause it to drop.