out attention. When it is considered that
open arcs for all-night burning were supplied with two pairs of
carbons, the second set going into use automatically when the first were
consumed, the value of the enclosed arc is apparent. However, the open
arc has served well and has given way to greater improvements. It is
rapidly disappearing from use.

The alternating-current arc-lamp was developed after the appearance of
the direct-current open-arc and has been widely used. It has no positive
or negative carbons, for the alternating current is reversing in
direction usually at the rate of 120 times per second; that is, it
passes through 60 complete cycles during each second. No marked craters
form on the tips and the two carbons are consumed at about the same
rate. The average temperature of the carbon tips is lower than that of
the positive tip of a direct-current arc, with the result that the
luminous efficiency is lower. These arcs have been made of both the open
and enclosed type. They are characterized by a humming noise due to the
effect of alternating current upon the mechanism and also upon the air
near the arc. This humming sound is quite different from the occasional
hissing of a direct-current arc. When soft carbons are used, the arc is
larger and apparently this mass of vapor reduces the humming
considerably. The humming is not very apparent for the enclosed
alternating-current arc. The alternating arc can easily be detected by
closely observing moving objects. If a pencil or coin be moved rapidly,
a number of images appear which are due to the pulsating character of
the light. At each reversal of the current, the current reaches zero
value and the arc is virtually extinguished. Therefore, there is a
maximum brightness midway between the reversals.

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 t