The Homopolar Motor...
click image for video
I made the coil by wrapping thick (14-16 swg) around a tube with a diameter just wider than an AA cell.
The Neodymium magnets are 15mm in diameter, a slightly larger diameter than the cell.
I used two magnets to get the height but it could have been 3 or 4 button magnets.
You can also use a larger cell like a C or D cell with correspondingly larger magnets.
The cell needs to be a NiCad, NiMH or Alkaline to provide the necessary current across the cell which is almost a direct short.
The motor works by creating a magnetic field in the wire, the +ve of the cell in this case is at the top, the current flows through the wire to the magnets (i.e. the -ve of the cell)
In the video the coil does not rotate until I touch it so that the bottom of the coil contacts the magnets.
You may see a small spark. N.B. The Cell will flatten easily.
As the coil wobbles intermittent contact is made and the magnetic field is created around the wire in the coil, it is repelled by the magnetism of the button cell and the induced field in the metal jacket of the cell. (The coil gets warm as it draws quite a lot of current that is the reason a cell with a low internal resistance is needed.)
Reversing the polarity of the magnets reverses the direction of rotation. As a point of interest go to Wikipedia.com and search for Maxwell's Left Hand Rule and Fleming's Left Hand Rule