A robot light bulb. (COMIC: The Good Soldier)
A light bulb was a device that used an electrical current to produce light.
If a round metal object were to be placed in between a light bulb and its socket, it would cause a surge and shut off the power. On Venus in 2400, Jeff Stone did this with one of his coat buttons while in the Dalek base in the Churchill Mountains, thus allowing their slaves to escape. (PROSE: The Secret of the Mountain) The Seventh Doctor did this with a coin when he and Ace were being held in the Savoy Hotel in an alternate 1951, thus allowing them to escape. (PROSE: Timewyrm: Exodus)
Brian Williams was once fiddling with a light bulb in Amy Pond and Rory Williams' house. (TV: Dinosaurs on a Spaceship)
Bernice Summerfield knew a joke involving light bulbs and Sontarans: "How many Sontarans does it take to change a light bulb? None, the bulb died a glorious death!" (PROSE: Sleepy)
A light bulb was also used as a symbol for new ideas. It was used in this way as an emoji: đĄ. (TV: Smile) Whenever the inhabitants of the Crooked World had new ideas, a glowing yellow light bulb appeared above their heads. These light bulbs were made by the Crooked World Electric Company. (PROSE: The Crooked World)
In the mid-20th century, the Cybermen of Mondas used robotic light bulbs with arms and legs. (COMIC: The Good Soldier)