Travel dial

A travel dial was a wrist-worn teleportation device used for travel on the planet Marinus. The wearer merely had to twist the dial in order to effect instantaneous travel to the pre-programmed destination.

Arbitan gave one each to the First Doctor, Ian, Barbara and Susan in order to make it easier for them to find the keys to the Conscience of Marinus. (DW: The Keys of Marinus)