Phenomenological Presentation of the Dirac Quantization of Magnetic Charge

The field-dependent difference in phase associated with two paths in an interference experiment employing electrically charged particles of electric charge $e$ is ordinarily proportional to the magnetic flux through a surface bounded by the difference of the paths. If considered as a visible electron-optical fringe displacement, it may be used as a phenomenological support for Dirac's condition $eg=\frac{1}{2}n$, restricting the strength of a magnetic monopole.