American electrical engineer who invented the mercury-vapour lamp (mercury lamp), a great advance in electrical lighting.

At an early age, Hewitt began research on electricity and mechanics in a greenhouse converted into a workshop. In 1901 he marketed his first mercury-vapour lamp (electric discharge lamp), but an improved model, brought out in 1903, had better colour qualities and found widespread use for industrial lighting. He later developed the quartz-tube mercury lamp, which found extensive use in biological research.

Other inventions by Hewitt include the mercury rectifier (a device for converting alternating current into direct current) and a radio receiver. He discovered the fundamental principle of the vacuum-tube amplifier during study of the flow of electricity through rarefied gases. His interest in aeronautics culminated in the construction (1918) of an early helicopter.