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By producing an arc between two carbon tips, a brilliant light that simulates natural sunlight is produced. In the recent past, these lights were open arcs, but as the incandescence gradually consumes the carbon tips, glass enclosed arc lamps replaced the open lamps to slow the burning of the carbon and thus make the lamps last longer. When preparing for indoor lighting for hydroponics when all the light bulbs may be shattered, carbon arc lamps may prove to be just the thing! At the present time no one is manufacturing open carbon arc lamps, but this simple technology could find a ready market in those wanting to be prepared for the troubled times ahead. These lamps work best in the direct current that alternative energies such as wind or water mills will furnish, and carbon is a readily available natural substance.

Encyclopedia Britannica gives the historical background on carbon arc lamps:

The incandescent lamp was not the first electric lamp; lighting devices employing an electric arc struck between electrodes of carbon had been developed and were in use earlier. Arc lamps were massive and complicated pieces of equipment that could be installed and maintained only by a skilled engineer. Used for street lighting, arc lamps had advantages, including reasonable reliability, high efficiency, and above all, a pleasant color, closely approximating natural sunlight. The light of the arc lamp was particularly kind to the color of the human complexion and the stone of historic buildings. Partly for this reason, these devices were retained in dignified city settings - for example, in the City of London - long after more modern light sources had come into wide use.

A device for producing light by maintaining an electric arc across a gap between two conductors; light comes from the heated ends of the conductors (usually carbon rods) as well as from the arc itself. Arc lamps are used in applications requiring great brightness, as in searchlights, large film projectors, and floodlights. ... Sir Humphry Davy constructed the first arc lamp (1807), using a battery of 2,000 cells to create a 4-inch (100 millimeter) arc between two charcoal sticks. When suitable electric generators became available in the late 1870's, practical use of arc lamps began. The Yablockhkov candle, an arc lamp invented by the Russian engineer Paul Yablochkov, was used for street lighting in Paris and other European cities from 1878.

Concise Encyclopedia of Science & Technology, by McGraw-Hill, describes the techology of arc lamps and in particular carbon arc lamps.

In most arc lamps the light results from the luminescence of the gas; however, in the carbon arc lamp the light is produced by the incandescence of one or both electrodes. ... The carbon arc lamp was the first practical commercial electric lighting device, but the use of arc lamps at present is limited. In many of its previous functions, the carbon arc lamp has been superseded by the high-intensity mercury vapor lamp.

An arc lamp whose electrodes are pure carbon. The lamp is either open, enclosed, or an intensified arc with incandescence at the electrodes and some light from the luminescence of the arc. The open arc form of the carbon arc is obsolete. In the enclosed type, a restricted supply of air slows the electrode consumption and secures a life approximately 100 h. ... In the high intensity arc the current may reach values of 125-150 A with positive volt-ampere characteristics, and these may operate directly from the line without ballast. ... Although carbon arc lamps are built for alternating-current operations, direct current produces a steadier and more satisfactory operation.

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