Applications of Electricity 231 



made strong and rigid by adding a layer of type metal to its under 

 side. 



Electrotyping has greatly reduced the expense, time, and labor 

 involved in printing books, magazines and newspapers. It allows 

 many plates to be made quickly and easily from a single page of 

 type. These plates may be used over and over again and, being 

 solid, are much preferable to pages set with individual type let- 

 ters which might fall out of place. The great high-speed rotary 

 newspresses could not operate without electrotype plates. To 

 this process, then, we owe the many inexpensive books, magazines 

 and newspapers which we enjoy today. 



Electric Heating. The fact that heat is produced in any 

 material through which electricity flows, and is more or less in- 

 tense according to the strength of the current and the resistance 

 it meets, has led to the invention of many electric heating de- 

 vices. Some of these are very convenient in household work and 

 are widely used. Electric heaters, electric toasters, electric flat- 

 irons, electric grills, electric waffle irons and the like are found in 

 many homes. In all of these devices the essential thing is a 

 coil or coils of wire which offer such resistance to the current 

 passing through that much heat is given off. Some metals have 

 much more resistance to a current than others. In household 

 devices wire made of nichrome is generally used, since it has a 

 high degree of resistance and hence becomes very hot. 



Electric Lighting. Electric lighting, as well as electric heat- 

 ing, depends on the principle of the transformation of energy. 

 When a substance is heated by electricity to a sufficiently high 

 degree of temperature it glows because a part of the heat energy 

 has been transformed into light energy. Taking advantage of 

 this fact, man has invented several devices to produce artificial 

 light. Among these are the arc light and the incandescent 

 light. 



The Arc Light. The arc light is made by placing two pieces 

 of hard carbon with their ends almost meeting and connecting 

 them in an electric circuit. The current is forced to jump the 

 gap between the ends of these carbons. Fine particles of carbon 

 pass across from one to the other, forming an arc over which 



