Control sphere

A control sphere was a device created by the Great Intelligence to control its Robot Yetis. It was a glass sphere that was fitted into the chest of a Yeti and served as the brain. It emitted a beeping noise. The Intelligence was able to control the Yeti and provide its motive power. Should the sphere be removed from the chest, it would move by itself to locate a deactivated Yeti and install itself into its chest. If the chest was blocked by something like a rock, the sphere would no longer think it is needed.

The Great Intelligence was also able to take the form of a three sided pyramid of control spheres. (TV: The Abominable Snowmen, The Web of Fear, HOMEVID: Downtime)

History
When "Jamie McCrimmon" gained his memories back, he heard several sounds, including the control sphere beeping. (AUDIO: Legend of the Cybermen)

Torchwood India obtained a Yeti control sphere from Tibet. It was later brought to the main Torchwood branch when Torchwood India was shut down in 1924. (AUDIO: Golden Age)

In 1999, the Ninth Doctor retrieved a "large silver metal ball" from the Mornington Crescent underground station, before exiting in the direction of Camden. (PROSE: Have You Seen This Man?)