Carbon arc lamps consist of two carbon rod electrodes in open air, supplied by a current-limiting ballast. The electric arc is struck by touching the rods then separating them. The ensuing arc heats the carbon tips to white heat. These lamps have higher efficiency than filament lamps, but the carbon rods are short lived and require constant adjustment in use. The lamps produce significant ultra-violet output, they require ventilation when used indoors, and due to their intensity they need protecting from direct sight.