A carbon arc lamp is a very old piece of technology - the first electric lights were carbon arcs!  The engineering dialogue of the 1800's and the early 1900's were filled with the discussion of arc lamps and improving their light.  Sadly, much of this data is lost today.  The arc lamp was invented in 1801 by Sir Humphrey Davy of England.  Even after the invention of the incandescent lightbulb in 1879, arc lamps were used when high powered lighting was necessary, and were in continuous use up through the 1980's.  This competition was the direct cause of the creation of General Electric!  In 1915, Sperry began producing high powered arc searchlights, and lights like these were instrumental in spotting aircraft at night in WWI and WWII.  Many stage artists and musicians had a requirement in their contract that they be lit by Super Trouper carbon arc spotlight, first manufactured in 1956.

Carbon arc lamps work by passing a high amperage current through two pieces of carbon, a cathode and an anode.  The high amperage involved causes the carbon to glow red hot then convert to plasma at the tip of the cathode, before arcing to the anode.  The brightest spot of the lamp is not the arc, but actually the plasma producing cathode tip, at least in DC systems.  In AC systems, the cathode and anode tips are equally bright (less so than DC).  This causes all sorts of problems with materials that you'd use in construction - the plasma is voracious in scavenging free electrons from the air and surrounding molecules, and the compounds produced by the arc are highly corrosive (ozone, gaseous acids).  It also makes a ton of both long and short wave UV - highly dangerous!

Huge thanks to Ed Paradis for all of these photos.  Check out his Flickr here!