An arc lamp produces light by the sparking (an electrical arc) of a high current between two conducting electrodes, usually carbon rods. English physicist Sir Humphry Davy invented the arc lamp in the early 1800s by using charcoal sticks and a battery with 2,000 cells to create an arc across a 4-inch (100 millimeter) gap. When suitable electric generators became available in the late 1870s, the practical use of arc lamps began. The Yablochkov candle, an arc lamp invented by the Russian engineer Pavel Yablochkov, was used for street lighting in Paris and other European cities starting in 1878.
A specimen from the late 19th century is pictured in the below tutorial.
In this lamp, light is produced when two carbon rods are connected to create a circuit of electrical current furnished (at the time this model was in operation) by a battery or dynamo. When the rods are then pulled apart slightly, the current jumps across the gap in an arch-like fashion, vaporized carbon acting as a kind of ionic bridge between the two rods.
Use the separation slider to do this yourself, first bringing the rods together, then pulling them apart just a bit.
The result is a bright light (Davy called it a “dazzling splendor”), its intensity due to the high resistance the current encounters as it struggles to jump between rods. The resistance produces in those carbon rod tips an intense heat measuring upwards of 6,000 degrees Fahrenheit (3,315 degrees Celsius), resulting in a white, incandescent brilliance (depicted for illustrative purposes as pink in this tutorial).
Getting this lamp to turn on is relatively simple. Getting it to stay on is another thing entirely. Consumed by the heat, the carbon rods gradually burn down. The distance of the gap between the rods must be just right to maintain the light-producing current (just see how particular the lamp in this tutorial is), so it must constantly be adjusted as the rods shrink. That was the hard and often labor-intensive part of this device, a predicament that spawned various, mostly imperfect solutions. In the example shown, an electromagnetic mechanism does the trick.
Today, arc lamps are used in applications requiring intense brightness, such as searchlights, floodlight and large film projector lights. The term arc lamp is usually restricted to lamps with an air gap between consumable carbon electrodes, but fluorescent and other electric discharge lamps generate light from arcs in gas-filled tubes. Some ultraviolet lamps are also of the arc type.