Various types of all these arcs have been developed to meet the different requirements of ordinary lighting and to adapt this method of light-production to the needs of projection, stage-equipment, lighthouses, search-lights, and other applications.

Up to this point the ordinary carbon arc has been considered and it has been seen that most of the light is emitted by the glowing end of the positive carbon.  In fact, the light from the arc itself is negligible.  A logical step in the development of the arc-lamp was to introduce salts in order to obtain a luminous flame.  This possibility as applied to ordinary gas-flames had been known for years and it is surprising that it had not been early applied to carbons.  Apparently Bremer in 1898 was the first to introduce fluorides of calcium, barium, and strontium.  The salts deflagrate and a luminous flame envelops the ordinary feeble arc-flame.  From these arcs most of the light is emitted by the arc itself, hence the name “flame-arcs.”

By the introduction of metallic salts into the carbons the possibilities of the arc-lamp were greatly extended.  The luminous output of such lamps is much greater than that of an ordinary carbon arc using the same amount of electrical energy.  Furthermore, the color or spectral character of the light may be varied through a wide range by the use of various salts.  For example, if carbons are impregnated with calcium fluoride, the arc-flame when examined by means of a spectroscope will be seen to contain the characteristic spectrum of calcium, namely, some green, orange, and red rays.  These combine to give to this arc a very yellow color.  As explained in a previous chapter, the salts for this purpose may be wisely chosen from a knowledge of their fundamental or characteristic flame-spectra.

These lamps have been developed to meet a variety of needs and their luminous efficiencies range from 20 to 40 lumens per watt, being several times that of the ordinary carbon open-arc.  The red flame-arc owes its color chiefly to strontium, whose characteristic visible spectrum consists chiefly of red and yellow rays.  Barium gives to the arc a fairly white color.  The yellow and so-called white flame-arcs have been most commonly used.  Flame-arcs have been produced which are close to daylight in color, and powerful blue-white flame-arcs have satisfied the needs of various chemical industries and photographic processes.  These arcs are generally operated in a space where the air-supply is restricted similar to the enclosed-arc principle.  Inasmuch as poisonous fumes are emitted in large quantities from some flame-arcs, they are not used indoors without rather generous ventilation.  In fact, the flame-arcs are such powerful light-sources that they are almost entirely used outdoors or in very large interiors especially of the type of open factory buildings.  They are made for both direct and alternating current and the mechanisms have been of several types.  The electrodes