hat
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 are
consumed rather rapidly so they are made as long as possible. In one
type of arc, the carbons are both fed downward, their lower ends forming
a narrow V wit