To prevent the top from overturning. As well as providing a force on the top as a whole, the magnetic field of the base gives a torque tending to turn its axis of spin. If the top were not spinning, this magnetic torque would turn it over. Then its south pole would point down and the force from the base would be attractive - that is, in the same direction as gravity - and the top would fall. When the top is spinning, the torque acts gyroscopically and the axis does not overturn but rotates about the (nearly vertical) direction of the magnetic field. This rotation is called precession (fig.2). With the LEVITRON®, the axis is nearly vertical and the precession is visible as a shivering that gets more pronounces as the top slows down. The effectiveness of spin in stabilizing a magnetically supported top such as the LEVITRON® was discovered by Roy M. Harrigan (4).