Dirac string

In physics, a Dirac string is a fictitiouos one-dimensional curve in space, stretched from a magnetic monopole to infinity. The gauge potential cannot be defined on the Dirac string, but it is defined everywhere else. The Dirac string acts as the solenoid in the Aharonov-Bohm effect, and the requirement that the position of the Dirac string should not be observable implies the Dirac quantization rule: the product of a magnetic charge and a electric charge must always be an integer multiple of $2\pi$.