by the screws, Y Y. The arc then is formed, and will continue to burn
steadily as long as the current remains constant. But the moment the
current falls, due to the increased resistance of the arc, a greater
proportion passes through the shunt, S' (Fig. 4), increasing its
magnetic moment on the iron core, while that of S is diminishing. The
result is that a moment arrives when equilibrium is destroyed, the iron
rod strikes smartly and sharply upon the spring, N T. Contact between T
and H is broken, and the current passes through the electromagnet of the
break in the lamp. The break is released for an instant, the carbons
approach each other. But the same rupture of contact introduces in the
shunt a new resistance of considerable magnitude (viz., 1,200 ohms),
that of the electromagnets of the break. Then the strength of the shunt
current diminishes considerably, and the solenoid, S, recovers briskly
its drawing power upon the rod, and contact is restored. The carbons
approach during these periods only about 0.01 to 0.02 millimeter.
If this is not sufficient to restore equilibrium it is repeated
continually, until equilibrium is obtained. The result is that the
carbon is continually falling by a motion invisible to the eye, but
sufficient to provide for the consumption of the carbons.

[Illustration: FIG. 6]

The contact between N T and H is never completely broken, the sparks are
very feeble, and the contacts do not oxidize. The resistances inserted
are so considerable that heating cannot occur, while the portion of the
current abstracted for the control is so small that it may be neglected.

The balance acts precisely like the key of a Morse machine, and the
break precisely like the sounder-receiver so well known in telegraphy.
It emits the same kind of sounds, and acts automatically like a skilled