486 



MAGNETISM AND ELECTRICITY 



speedometer of an automobile. (Figure 157.) The light is made 

 to glow. 



In the preceding experiment we found that electrical 

 energy, in overcoming the resistance of the iron wire, was 



changed into heat. When a 

 current of electricity passes 

 through any substance, the sub- 

 stance offers resistance to it. 

 The amount of resistance offered 

 by a conductor varies with the 

 kind of material, its length and 

 FIGURE 156 its thickness. Heating due to 



resistance of an electric current is utilized in 

 the construction of electric flatirons, toasters, 

 stoves, and other devices. The electricity is 

 generally conducted to the utensils through a 

 wire made up of a number of small copper wires, 

 covered with non-conducting materials. The 

 resistance of the connecting cord is very low. 



From this cord, the current 

 passes through coils in the 

 utensil that offer high re- 

 sistance. These are so ar- 

 ranged that the resulting 

 heat is delivered with al- 

 most no loss to the surface 

 which is to be heated. Al- 

 though it costs more to 

 produce the same amount 

 of heat by electricity than 

 it does by the other methods usually employed in the home, 

 yet for many purposes this heat can be applied with so 



FIGURE 157 



ELECTRIC IRON SHOWING HEATING 

 ELEMENT (E) 



