Peter Cooper Hewitt (1861-1921), US electrical engineer, holding a piece of equipment. Hewitt is famous for his mercury-vapour lamp (patented 1901) and for building the first mercury arc rectifier (in 1902). Early models of the lamp were a thin tube with a pool of mercury filling the lamp with mercury vapour. Current flowing between electrodes and through the mercury vapour caused a discharge of light. The light produced was bluish-green, but could be used in areas like photography, which was black-and-white and did not need accurate colours. The mercury arc rectifier was used to convert alternating current to direct current, and was used in industry and on the railways. Photographed between 1901 and 1910.