The annexed figure of the plain lamp will convey an understanding of
its general appearance. The upper carbon is fed downward by the
mechanism contained in the box above, and is carried by a vertical
round rod called the carbon holding rod.

[Illustration: THE THOMSON ARC LAMP.]

In the regulating box of the lamp there exists a simple mechanism, the
result of careful study and experiment to discover the best and
simplest combination of appliances, which would obviate the necessity
for the use of clockwork or dash-pots, from which fluids might be
accidentally spilled, for obtaining a gradual feeding of the carbon as
fast as it is consumed in producing the light, and at the same time to
maintain the arc or space between the carbons in burning, of such
extent as to give a steady, noiseless light, of greatest possible
economy.

The lamp, once adjusted, does not require any readjustment, and, in
fact, is built in such a manner as to avoid the presence of adjusting
devices in it. The lamp also contains an automatic safety device for
preserving the continuity of the circuit in case of accidental injury
to the feeding mechanism or the carbons of the lamps. This is quite
important when a considerable number of lights are operated upon one
circuit wire, as a break in the circuit, due to a defective lamp,
would result in the extinguishment of all the lights. With the safety
device mentioned, such a break does not occur, but the flow of current
is preserved through the faulty lamp.

By an exceedingly simple device upon the carbon holding rod, the lamps