of this
transference, and the heat thus developed is sometimes so great that
fire occurs. For this reason, wires are heavily insulated and extra
protection is provided at points where numerous wires touch or cross.

Conductors and insulators are necessary to the efficient and economic
flow of a current, the insulator preventing the escape of electricity
and lessening the danger of fire, and the conductor carrying the
current.

300. The Telegraph. Telegraphy is the process of transmitting
messages from place to place by means of an electric current. The
principle underlying the action of the telegraph is the principle upon
which the electric bell operates; namely, that a piece of soft iron
becomes a magnet while a current flows around it, but loses its
magnetism as soon as the current ceases.

In the electric bell, the electromagnet, clapper, push button, and
battery are relatively near,--usually all are located in the same
building; while in the telegraph the current may travel miles before
it reaches the electromagnet and produces motion of the armature.

[Illustration: FIG. 217.--Diagram of the electric telegraph.]

The fundamental connections of the telegraph are shown in Figure 217.
If the key _K_ is pressed down by an operator in Philadelphia, the
current from the battery (only one cell is shown for simplicity) flows
through the line to New York, passes through the electromagnet _M_,
and thence back to Philadelphia. As long as the key _K_ is pressed
down, the coil _M_ acts as a magnet and attracts and holds fast the
armature _A_; but as soon as _K_ is released, the current is broken,
_M_ loses its magnetism, and the armature is pulled back by the spring
_D_. By a mechanical device, tape is drawn uniformly under the light
marker _P_ attached to the armature. If _K_ is closed for but a short
time, the armature is drawn down for but a short interval, and the
marker registers a dot on the tape. If _K_ is closed for a longer
time, a short dash is made by the marker, and, in general, the length
of time that _K_ is closed determines the length of the marks recorded
on the tape. The telegraphic alphabet consists of dots and dashes and
their various combinations, and hence an interpretation of the dot and
dash symbols recorded on the tape is all that is necessary for the
receiving of a telegraphic message.

The Morse telegraphic code, consisting of dots, dashes, and spaces, is
given in Figure 218.

[Illustration: