230 Common Science 



We have talked about making electricity work when 

 it flows in a steady stream, and everybody knows that 

 it makes lights glow, makes toasters and electric stoves 

 hot, and heats electric irons. But did it ever strike 

 you as remarkable that the same electricity that flows 

 harmlessly through the wires in your house withcut 

 heating them, suddenly makes the wire in your toaster or 

 the filament in your incandescent lamp glowing hot? 

 The insulation is not what keeps the wire cool, as you 

 can see by the next experiment. 



Experiment 69. Between two of the laboratory switches 

 you will find one piece of wire which has no insulation. 

 Turn on the electricity and make the lamp glow ; see that 

 you are standing on dry wood* and are not touching any 

 pipes or anything connected to the ground. Feel the bare 

 piece of wire with your fingers. Why does this not give 

 you a shock? What would happen if you touched your 

 other hand to the gas pipe or water pipe? Do not try it! 

 But what would happen if you did ? 



The reason that the filament of the electric lamp gets 

 white hot while the copper wire stays cool is this : All 

 substances that conduct electricity resist the flow some- 

 what ; there is something like friction between the wire 

 and the electricity passing through it. The smaller 

 around a wire is, the greater resistance it offers to the 

 passing of an electric current. The filament of an elec- 

 tric lamp is very fine and therefore offers considerable 

 resistance. However, if the filament were made of 

 copper, even as fine as it is, it would take a much greater 

 flow of electricity to make it white hot, and it would 

 be very expensive to use. So filaments are not made of 



