Experimental apparatus used to observe and determine a value of the magnetic flux quantum in a high critical temperature ceramic superconductor, yttrium-barium-copper oxide (Y- Ba2- Cu3-O7-x). The superconductor ring (centre, dark grey) was cooled in liquid helium. A magnetic flux applied by a solenoid (right) was monitored by a superconducting quantum interference device (SQUID) magnetometer (left). The value of the flux quantum thus determined implies that the charge carriers of superconductivity are electron pairs and provides a dramatic demonstration of quantum mechanics on a macroscopic scale. A paper describing this experiment was first published by Gough et al in Nature, volume 326,