The gun, developed by the Third Doctor. (TV: Invasion of the Dinosaurs)
A stun gun was created by the Third Doctor, designed to incapacitate the dinosaurs that were appearing in London in the 1970s. It worked by temporarily neutralising brain cells, a technique which the Doctor described would not be known for centuries on Earth.
Aware that, with a dinosaur in capture, the Doctor would be able to find his base, Whitaker told Mike Yates to fix a device he handed him to the gun, which would render it useless. He did so and, when the Doctor attempted to use it on an apatosaurus, it had no effect.
Whitaker used a time eddy to produce a Tyrannosaurus Rex nearby in a bid to kill the Doctor. Unwilling to let him die, Yates picked up the gun, removed the device he had attached and rendered the creature senseless. (TV: Invasion of the Dinosaurs)
