Harry Grindell Matthews crouched inside a small cupboard and placed one hand on the lever of his new invention. The air crackled with electricity. On the other side of his laboratory, three representatives of the Air Ministry, Matthews’ two assistants and his business manager waited for the signal.

Thick linoleum covered the chequerboard floor at 2 Harewood Place, London. From the outside, the tall red-brick building with a view of Oxford Street looked more like a block of mansion flats than a scientific workshop. It was May 26 1924, a Monday morning.

The men stared at the contraption. It had the appearance of a metal spotlight, 4ft across with a ceramic base and three smaller cones mounted around its rim. It was pointed at a single-cylinder motorcycle engine on the far side of the laboratory. Harnessing the power of light and the new technology of radio waves, Matthews said, his beam would stop the engine dead.