One of my all-time favorite "mad scientists", Harry Grindell Matthews was a fascinating and talented inventor of many devices, including an alleged "Death Ray".
While serving in the British military during the Boer War in 1899, he developed an interest in the new and experimental field of wireless telephony. Within a year, he had devised a wireless communication system that he demonstrated for King George V at Buckingham Palace.
By 1921 Matthews was recording sound tracks using light (technology that foreshadowed the compact disc by half a century), and made a sound film of explorer Sir Ernest Shackleton before his ill-fated final departure for the Antarctic. (Matthews credited himself with having invented the sound film, but William Kennedy Dickson had already done it in 1895.)
In 1923 Matthews made worldwide headlines with his announcements of having developed a Death Ray. According to Wikipedia:
The War Office contacted Matthews in February 1924 to request a demonstration of his ray. Matthews did not answer to them but spoke to journalists and demonstrated the ray to a Star reporter by igniting gunpowder from a distance. He still refused to say how the ray actually worked, just insisted that it did. When the British government still refused to rush to buy his ideas, he announced that he had an offer from France.
The Air Ministry was wary, partially because of previous bad experiences with would-be inventors. Matthews was invited back to London to demonstrate his ray on April 26 to the armed forces. In Matthews's laboratory they saw how his ray switched on a light bulb and cut off a motor. He failed to convince the officials, who also suspected trickery or a confidence game. When the admiralty requested further demonstration, Matthews refused to give it.
In July 1924, Matthews left for the USA to market his invention. When he was offered $25,000 to demonstrate his beam to the Radio World Fair at Madison Square Garden, he again refused and claimed, without foundation, that he was not permitted to demonstrate it outside England. US scientists were not impressed. One Professor Woods offered to stand in front of the death ray device to demonstrate his disbelief. Regardless, when Matthews returned to Britain, he claimed that the USA had bought his ray but refused to say who had done it and for how much.
In 1927 he developed a ‘Sky Projector’, for projecting images onto the clouds. It's widely believed that the iconic Bat-Signal in Batman comics was inspired by Matthews' invention.
Other inventions and ideas variously attributed to Harry include: an auto-pilot for aeroplanes, automatic street lamps that came on at dusk, and an ahead-of-its-time liquid hydrogen rocket fuel for space travel.
Harry died of a heart attack on September 11, 1941.