In 1924 he developed a mechanical television system to transmit moving images by means of electrical signals, which he demonstrated on 25 March 1925 at a London department store, Selfridges.
Due to the low sensitivity of the photoelectric cells, Baird's first system was not able to televise human faces, because they had inadequate contrast.
Baird therefore used a ventriloquist's dummy, the brightly painted face of which had greater contrast, and made it move and talk before the scanner.
The incandescent lights illuminating the subject to be televised also generated so much heat that Baird could not use a human for the testing.
Stooky Bill was a major plot point in the Doctor Who 60th anniversary special "The Giggle", in which he was used by the villainous Toymaker to spread insanity through every screen on Earth.