  In type-I superconducting cylinders bulk superconductivity is destroyed above
the first critical current. Below the second critical current the `type-I mixed
state' displays fluctuation superconductivity which contributes to the total
current. A magnetic flux on the axis of the cylinder can change the second
critical current by as much as 50 percent so that half a flux quantum can
switch the cylinder from normal conduction to superconductivity: the
Aharonov--Bohm effect manifests itself in macroscopically large resistance
changes of the cylinder.
