If one supposes that a particle with a single magnetic
pole can exist and that it interacts with charged particles,
the laws of quantum mechanics lead to the requirement
that the electric charges shall be quantized—all charges
must be integral multiples of a unit charge e connected
with the pole strength g by the formula eg = ihc. Since
electric charges are known to be quantized and no reason
for this has yet been proposed apart from the existence
of magnetic poles, we have here a reason for taking mag-
magnetic poles seriously. The fact that they have not yet been
observed may be ascribed to the large value of the quantum
of pole.
In 1931 I gave a primitive theory which described the
motion of a pole in the field of a charged particle whose
motion is given, or the motion of a charged particle in the
field of a pole whose motion is given. The present paper
sets up a general theory of charged particles and poles in
interaction through the medium of the electromagnetic
field. The idea which makes this generalization possible
consists in supposing each pole to be at the end of an un-
observable string, which is the line along which the electro-
electromagnetic potentials are singular, and introducing dy-
dynamical coordinates and momenta to describe the motion
of the strings. The whole theory then comes out by the
application of standard methods. There are unsolved
difficulties, concerned with the interaction of a point
charge or a point pole with the field it produces itself,
such as occur in all dynamical theories of fields and par-
particles in interaction.