  The static interaction of a point charge and a polarisable particle and
between two polarisable particles is discussed in vacuo, and force and energy
considerations are made. In particular a critical distance is shown (in
principle) to appear in the two-dipole case, where the polarisation is self-
sustained, and above which it disappears and below which it tends to explode.
In the case of a polarisable particle with a nonzero charge interacting with a
charge (of the same sign) there is a distance where repulsion and attraction
are balanced.
