CHAPTER XXXI 



SOME USES OF ELECTRICITY 



288. Heat. Any one who handles electric wires knows 

 that they are more or less heated by the currents which flow 

 through them. If three cells are arranged as in Figure 200 

 and the connecting wire is coarse, the heating of the wire is 

 scarcely noticeable; but if a shorter wire of the same kind 

 is used, the heat produced is slightly greater; and if the 

 coarse wire is replaced by a short, fine wire, the heating of 

 the wire becomes very marked. We are accustomed to say 

 that a wire offers resistance to the flow of a current; that 

 is, whenever a current meets resistance, heat is produced 

 in much, the same way as when mechanical motion meets an 

 obstacle and spends its energy in friction. The flow of elec- 

 tricity along a wire can be compared to the flow -of water 

 through pipes : a small pipe offers a greater resistance to the 

 flow of water than a large pipe ; less water can be forced 

 through a small pipe than through a large pipe, but the fric- 

 tion of the water against the sides of the small pipe is much 

 greater than in the large one. 



So it is with the electric current. In fine wires the resist- 

 ance to the current is large and the energy of the battery is 

 expended in heat rather than in current. If the heat thus 

 produced is very great, serious consequences may arise ; for 

 example, the contact of a hot wire with wall paper or dry 

 beams may cause fire, Insurance companies demand that 

 the wires used in wiring a building for electric lights be of a. 



312 



