Superconductivity remained an intriguing but obscure effect for many years after its discovery. The work at Leiden had established the most spectacular property of a superconductor: its electrical resistance is zero. By contrast a normal metal shows an electrical resistance. It is impossible to measure a zero resistance; however it is now known that a dc current flowing round a superconducting ring remains unchanged for years. This persistence places an upper limit on the electrical resistivity of at least 1015 times smaller than that of a normal metal at the same temperature. No voltage is observable across a superconductor when it is included in an otherwise normal (i.e. non-superconducting) circuit and a direct current is flowing: for all practical purposes the resistance is zero.