What did Uncle Andrew discover?
Uncle Andrew discovers a way to travel between worlds. His godmother, Mrs. Lefay, gave him a box of magic dust. After many attempts and trials, Uncle Andrew uses the magic dust to create sets of rings with magical properties. The gold ring transports the wearer to a magical world; to return, Andrew fashioned a companion green ring. Andrew knows that the gold rings work, because he tested it on some unfortunate guinea pigs. They disappeared, but not having the ability to change rings, could not reappear to confirm Andrew's success.
At the beginning of The Magician's Nephew, Andrew tricks Diggory's friend Polly into touching the gold ring. When she disappears, Diggory has no choice but to take the second set of rings to rescue her.