In 1848 the first electric arc lamp used for general lighting was installed in Paris.  It was supplied with current by a large voltaic cell, but the success of the electric arc was obliged to await the development of a more satisfactory source of electricity.  A score of years was destined to elapse, after the public was amazed by the first demonstration, before a suitable electric dynamo was invented.  With the advent of the dynamo, the electric arc was rapidly developed and thus there became available a concentrated light-source of high intensity and great brilliancy.  Gradually the size was increased, until at the present time mirrors as large as seven feet in diameter and electric currents as great as several hundred amperes are employed.  The beam intensities of the most powerful search-lights are now as great as several hundred million candles.

The most notable advance in the design of arc search-lights was achieved in recent years by Beck, who developed an intensive flame carbon-arc.  His chief object was to send a much greater current through the arc than had been done previously without increasing the size of the carbons and the unsteadiness of the arc.  In the ordinary arc excessive current causes the carbons to disintegrate rapidly unless they are of large diameter.  Beck directed a stream of alcohol vapor at the arc and they were kept from oxidizing.  He thus achieved a high current-density and much greater beam intensities.  He also used cored carbons containing certain metallic salts which added to the luminous intensity, and by rotation of the positive carbon so that the crater was kept in a constant position, greater steadiness and uniformity were obtained.  Tests show that, in addition to its higher luminous efficiency, an arc of this character directs a greater percentage of the light into the effective angle of the mirror.  The small source results in a beam of small divergence; in other words, the beam differs from a cylinder by only one or two degrees.  If the beam consisted entirely of parallel rays and if there were no loss of light in the atmosphere by scattering or by absorption, the beam intensity would be the same throughout its entire length.  However, both divergence and atmospheric losses tend to reduce the intensity of the beam as the distance from the search-light increases.

Inasmuch as the intensity of the beam depends upon the actual brightness of the light-source, the brightness of a few modern light-sources are of interest.  These are expressed in candles per square inch of projected area; that is, if a small hole in a sheet of metal is placed next to the light-source and the intensity of the light passing through this hole is measured, the brightness of the hole is easily determined in candles per square inch.

BRIGHTNESS OF LIGHT-SOURCES IN CANDLES PER SQUARE INCH