In our city streets, the overhead wires are supported on glass knobs or are closely wrapped, in order to prevent the escape of electricity through the poles to the ground.  In order to have a steady, dependable current, the wire carrying the current must be insulated.

Lack of insulation means not only the loss of current for practical uses, but also serious consequences in the event of the crossing of current-bearing wires.  If two wires properly insulated touch each other, the currents flow along their respective wires unaltered; if, however, two uninsulated wires touch, some of the electricity flows from one to the other.  Heat is developed as a result 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.