This Demonstration is concerned with the interaction between a classical electric charge and a magnetic monopole. The problem was actually solved a long time ago by Poincaré [3] (see also [4] and [5]). For historical continuity, we use Gaussian electromagnetic units. The field of a point monopole of magnetic charge is given by , in complete analogy with Coulomb's law for an electric charge. The Lorentz force on an electron of mass and charge moving with velocity is then , leading to Newton's equation of motion .

The equations of motion can be completely solved with a few vector operations. Taking the scalar product with , we have ; therefore , so that the speed of the electron is a constant (for ). Taking the scalar product with gives , so that , with the solution , where , the initial separation of the electron and monopole and .

Taking the vector product of with Newton's equation, we find . The orbital angular momentum is evidently not a constant of the motion (although its magnitude is). Note also that . Instead, the appropriate constant of the motion is the vector (somewhat reminiscent of the Runge–Lenz vector for the Coulomb problem). The angular momentum of the electromagnetic field can be calculated from , a result first obtained by J. J. Thomson. Thus the vector , sometimes called the Poincaré vector, represents the total angular momentum: mechanical plus electromagnetic. It is shown by a blue arrow. Since is perpendicular to , . Also, , thus the trajectory of the position vector is evidently confined to the surface of a right circular cone (the Poincaré cone) with constant slant angle with respect to the axis .

The motion of about the origin with an angular velocity determines the -dependence of the trajectory. We have , so that integration gives

The electron spin is, in its lowest energy state, parallel everywhere to the magnetic field and does not contribute to the motion.

In the graphic, all variables are scaled relative to . The monopole is marked with a blue cross, while the electron's trajectory is show in red. The trajectory is actually a geodesic on the surface of the cone, which would follow a straight line if the cone were unrolled. To keep within the scale of the diagram, the values of are limited to be between 1.001 and 1.005 (multiples of ).