ELECTRIC LIGHTING 



487 



FIGURE 158 



little loss that the use of electricity in some kinds of heating 

 becomes not only convenient but also really economical. 



Heat generated by electricity is also 

 used for welding (Figure 158), and is 

 beginning to replace the forge. If metal 

 rods are pressed together end to end and 

 a sufficiently great current of electricity 

 is sent through them, the heat generated 

 at the point of contact, where the resist- 

 ance is greatest, will be sufficient to weld 

 them together. The rails of car tracks 

 are often welded together in this way. 



Wherever electricity is received from wires in which the 

 strength of the current may vary considerably 

 from time to time, it is necessary to protect 

 electrical appliances from the heat caused by 

 too great a current. This is done by inserting 

 in the circuit a wire which will melt if too 

 much current passes through it, and will thus 

 instantly break the circuit. Such a safety de- 

 vice is called &fme. (Figure 159.) 



Electric Lighting. The little electric lamp 

 used in Experiment 157, like most other in- 

 candescent lamps, consists of a thread or fila- 

 ment of carbon inclosed in a glass bulb from 

 which the air has been exhausted. When this 

 lamp is connected with an electric current the 

 carbon is heated white hot by the resistance it 

 offers to the electric current. The carbon cannot burn be- 

 cause there is no air in the bulb, and it does not melt since 

 there is not sufficient heat to accomplish this. Incandescent 



FIGURE 159 



TUNGSTEN 

 LAMP 



