An infinitesimally thin solenoid carrying quantized magnetic flux ${\mathrm{\Phi }}_{\mathit{n}}$=nbc/e cannot be detected by point particles. However, if encircled by their world sheets, such a solenoid need not stay invisible to charged superconducting closed strings. A novel reparametrization-invariant effect, sensitive to the total (gauge-invariant) magnetic flux penetrating through the test loop, is derived. Borrowing the Kaluza-Klein language, it reflects the interplay between the σ periodicity of the world sheet and the ${\mathit{x}}^5$ periodicity of space-time.

• Received 30 March 1990

DOI:https://doi.org/10.1103/PhysRevLett.65.145