You have two bars of iron. One is magnetized along its length; the other is not. Without using any other instrument (thread, filings, other magnets, etc.), find out which is which.
Answer
Bar magnets are “dead” in their centres (i.e. there is no magnetic force, since the two poles cancel out). So, if bar A is the magnet, then bar B won’t stick to its centre.
However, bar magnets are quite “alive” at their edges (i.e. the magnetic force is concentrated). So, if bar B is the magnet, then bar A will stick nicely to its end.
