Arc lamps were first demonstrated in the 1840s and by 1870, dynamo-electric machinery made it posible to sustain them for indefinite periods. The strong light produced by the arc lamp suited the illuminating of large outdoor areas, but as most arc lamps operated in free air, the carbon rods soon burnt away, needing frequent renewal. In the Jandus enclosed arc lamp, however, the flow of air was restricted by a glas cover. The carbons lasted longer, giving up to 100 hours' light, but a sooty deposit on the glas reduced brightnes.
© Science Museum / Science & Society Picture Library