Dirac's magnetic monopole (figure).

<p>A schematic representation of Dirac’s magnetic monopole. The monopole is imagined to be the end point of a semi-infinitely long, infinitesimally thin solenoid known as a "Dirac string", here shown from the side (red lines). The magnetic field lines (black arrows) are shown emanating from the point at the end of the string (imagine the red lines are infinitely close together). If such Dirac monopoles exist, it is the requirement that the string is undetectable that means electric charge must be quantised. Image credit: The Institute for Research in Schools 2016.</p> <p>CERN@school DRN: CAS-PUB-MDL-000007-v1.0</p>