THE TELEGRAPH 221 



the poles to the ground. In order to have a steady, depend- 

 able 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 re- 

 spective 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 thfc 

 escape of electricity and lessening the danger of fire, and the 

 conductor carrying the current. 



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 to flow around it. 



In the electric bell, the electromagnet, clapper, push button, 

 and battery are relatively near, usually all are located in 

 the same building. In the telegraph the current may travel 

 miles before it reaches the electromagnet and produces motion 

 of the armature. 



The fundamental connections of the early telegraph are shown 

 in Figure 91. 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 then back to Phila 



