oving-picture
production studios, in moving-picture projection, and in certain aspects
of stage-lighting is firmly established, and it appears that they will
find application in certain chemical industries because the arc is a
powerful source of radiant energy which is very active in its effects
upon chemical reactions.

The luminous efficiencies of arc-lamps depend upon so many conditions
that it is difficult to present a concise comparison; however, the
following may suffice to show the ranges of luminous output per watt
under actual conditions of usage. These efficiencies, of course, are
less than the efficiencies of the arc alone, because the losses in the
mechanism, globes, etc., are included.

Lumens per watt
Open carbon arc 4 to 8
Enclosed carbon arc 3 to 7
Enclosed flame-arc (yellow or white) 15 to 25
Luminous arc 10 to 25

Another lamp differing widely in appearance from the preceding arcs may
be described here because it is known as the mercury-arc. In this lamp
mercury is confined in a transparent tube and an arc is started by
making and breaking a mercury connection between the two electrodes. The
arc may be maintained of a length of several feet. Perhaps the first
mercury-arc was produced in 1860 by Way, who permitted a fine jet of
mercury to fall from a reservoir into a vessel, the reservoir and
receiver being connected to the poles of a battery. The electric current
scattered the jet and between the drops arcs were formed. He exhibited
this novel light-source on the mast of a yacht and it received great
attention. Later, various investigators experimented on the production
of a mercury-arc and the first successful ones were made in the form of
an inverted U-tube with the ends filled with mercury and the remainder
of the tube exhausted.

Cooper Hewitt was a successful pioneer in the production of practicable
mercury-arcs. He made them chiefly in the form of straight tubes of
glass up to several feet in length, with enlarged ends to facilitate
cooling. The tubes are inclined so that the mercury vapor which
condenses will run back into the enlarged end, where a pool of mercury
forms the negative electrode. The arc may be started by tilting the tube
so that a mercury thread runs down the side and connects with the
positive electrode of iron. The heat o