Arc lamp

The arc lamp produces light by the sparking (or arcing, from voltaic arc) of a high current between two carbon rod electrodes. The rods are touched and then slowly drawn apart; as the rods separate the current is "struck" and arcs across the gap in a bright, ionized path. The arc produces a temperature of several thousand degrees, and the tips of the carbon rods are heated to incandescence, creating light. The rods are slowly vaporized during the process and need to be regularly adjusted to maintain the arc. The concept was first demonstrated by Sir Humphry Davy in the early 19th century (1802, 1805, 1807 and 1809 are all mentioned), using charcoal sticks and a 2000-cell battery to create a arc across a 4-inch gap.

The concept was improved upon by a number of people including William Staite and Charles F. Brush. There were attempts to produce the lamps commercially after 1850 but the lack of a constant mains electricity supply thwarted efforts. It was not until the 1870s that lamps such as the Yablochkov candle were more commonly seen. The harsh and brilliant light was found most suitable for public areas, being around 200 times more powerful than contemporary filament lamps. There were two major advances in the 1880s when the arcs were enclosed in a small tube to slow the carbon consumption (increasing the life span to around 100 hours) and with the introduction of flame arc lamps, where the carbon rods had added salts (usually magnesium, strontium, barium or calcium fluorides) to increase light output and produce different colours.

The arc lamps were soon superseded by the more efficient and longer-lasting filament lamps in most roles, remaining in only certain niche markets such as cinema projection and search lights.\n