The classical dynamics of two interacting particles of equal mass and
equal or opposite charge, moving in a plane and a perpendicular magnet
ic field, is discussed. The simplest trajectories are similar to those
of a single particle in the presence of crossed electric and magnetic
fields (Hall configuration). Such motion occurs over a ribbon that ma
y be straight (for opposite charges), or bent into a circle (for ident
ical particles). (C) 1997 American Association of Physics Teachers.