it,
there is an actual break of very small extent. Suppose that to the ends
of the wires leading from a dynamo's terminals we attach two carbon
rods, and touch the end of the rods together. The tips become white hot,
and if they are separated slightly, atoms of incandescent carbon leap
from the positive to the negative rod in a continuous and intensely
luminous stream, which is called an _arc_ because the path of the
particles is curved. No arc would be formed unless the carbons were
first touched to start incandescence. If they are separated too far for
the strength of the current to bridge the gap the light will flicker or
go out. The arc lamp is therefore provided with a mechanism which, when
the current is cut off, causes the carbons to fall together, gradually
separates them when it is turned on, and keeps them apart. The principle
employed is the effort of a coil through which a current passes to draw
an iron rod into its centre. Some of the current feeding the lamp is
shunted through a coil, into which projects one end of an iron bar
connected with one carbon point. A spring normally presses the points
together when no current flows. As soon as current circulates through
the coil the bar is drawn upwards against the spring.

SERIES AND PARALLEL ARRANGEMENT OF LAMPS.

When current passes from one lamp to another, as in Fig. 82, the lamps
are said to be in _series_. Should one lamp fail, all in the circuit
would go out. But where arc lamps are thus arranged a special mechanism
on each lamp "short-circuits" it in case of failure, so that current may
pass uninterruptedly to the next.

[Illustration: FIG. 82.--Incandescent lamps connected in "series."]

Fig. 83 shows a number of lamps set _in parallel_. One terminal of each
is attached to the positive conductor, the other to the negative
conductor. Each lamp therefore forms an independent bridge, and does
not affect the efficiency of the rest. _Parallel series_ signifies a
combination of the two systems, and would be illustrated if, in Fig. 83,
two or more lamps were connected in series groups from one conductor to
the other. This arrangement is often used in arc lighting.

[Illustration: FIG. 83.--Incandescent lamps connected in "parallel."]

CURRENT FOR ELECTRIC LAMPS.

This may be either direct or alternating. The former is commonly used
for arc lamps, the latter for incandescent, as it is easily stepped-down
from the high-pressure mains for use in a hou