322 MODERN ELECTRICAL INVENTIONS 



the current again flows. Substances which, like an air gap, 

 interfere with the flow of electricity are called non-conduc- 

 tors, or, more commonly, insulators. Substances which, like 

 the earth, the human body, and all other moist objects, con- 

 duct electricity are conductors. If the telephone and electric 

 light wires in our houses were not insulated by a covering of 

 thread, or cloth, or other non-conducting material, the elec- 

 tricity would escape into surrounding objects instead of flow- 

 ing through the wire and producing sound and light. 



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 carry- 

 ing 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 trans- 

 mitting messages from place to place by means of an electric 

 current. The principle underlying the action of the tele- 

 graph is the principle upon which the electric bell operates ; 



