Chumbley
Chumblies were the nickname Vicki gave to the Rills' small robot servants, after the "wobbly" sound they made when moving.
The Doctor deduced they were blind because they seemed to feel their way around the exterior of the TARDIS. Later, he determined that they did have the ability to detect sound waves and heat. They were also equipped with a weapon that could project flames. They were powered by a form of magnetism, and could be immobilized by ensnaring them in a metal mesh. They resisted gunfire by deactivating and then reactivated when the firing stopped. Upon deactivation, their bodies telescoped down, reducing their height roughly in half. The concentric rings of metal that comprised their bodes could thus fold down into each other, to protect the interior of the robot.
Their primary task was to act as a translator between the Rills and other beings. They could analyze the speech patterns of other beings and allow the thoughts of the Rills to be "spoken" through their onboard speakers in the language of the recipient. They continued to appear around the Drahvins' ship because the Rills were merely trying to communicate.
The Chumbleys assisted the Rills in a variety of other ways. Because the Rills could not metabolize oxygen, the Chumbleys were the only way in which the Rills could experience oxygen-rich planets, such as the one on which they had crashed. They were those a kind of probe, sending a variety of data back to their masters.
Vicki was the first person on the planet to discover their true purpose. When she told the Rills she had named the robots "Chumblies", they did not object. Indeed, they were quite happy to use the term themselves. (DW: Galaxy 4)
Behind the scenes
According to Jeremy Bentham, the Chumblies were designed by Richard Hunt. Like Daleks, they required a performer to operate the machinery from inside. However, their small, three-foot stature meant that those performers all had to be dwarfs. One of them was Angelo Muscat, who would later play the butler in The Prisoner. The action of the Chumbley's telescoping down into a "closed" position when deactivated made the prop too small for any performer. Thus, a "performer-less" prop was constructed for filming the "down" position.
