Arc lamp, late 19th century.
© Science Museum / Science & Society Picture Library
Arc lamp with mechanism controlled by an electromagnet. Arc lamps were first demonstrated in the 1840s and by 1870, dynamo-electric machinery could provide the electricity to sustain them for long periods. The strong light produced by the arc lamp suited the illuminating of large outdoor areas, but as the arc operated in air the carbon rods burnt away, and needed frequent renewal. A mechanism operated by electromagnets controlled the position of the upper carbon rod to keep the gap between the carbons constant.