Dirac string
In physics, a Dirac string is a fictitious one-dimensional curve in space, stretched from a magnetic monopole - also called the Dirac 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 an electric charge must always be an integer multiple of 2π.