242 Common Science 



on the electricity. What happens? Examine the pieces of 

 the fuse wire that are left. 



It was so easy for the electricity to pass through the 

 nails and wire, that it gushed through at a tremendous 

 rate. This melted the fuse wire, or blew out the fuse. 

 If the fuse across the gap by the socket had not been the 

 more easily burned out, one or perhaps both of the more 

 expensive fuses up above, where the wire comes in, 

 would have blown out. These cost about 10 cents each 

 to replace, while the fuse wire you burned out costs 

 only a fraction of a cent. If there were no fuses in the 

 laboratory wirings and you had " short circuited " the 

 electricity (given it an easy enough path), it would have 

 blown out the much more expensive fuses where the 

 electricity enters the building. If there were no big 

 fuses where the electricity enters the building, the rush 

 of electricity would make all the copper wires through 

 which it flowed inside the building so hot that they would 

 melt and set fire to the building. As long as you keep 

 a piece of fuse wire across the gap, there is no danger 

 from short circuits. 



Why fuse wire melts. For two reasons, the fuse wire 

 melts when ordinary wire would not. First, it has 

 enough resistance to electricity so that if many amperes 

 (much current) flow through, it gets heated. It has 

 not nearly as much resistance, however, as the filament 

 in an electric lamp or even as has the long resistance 

 wire. It does not become white hot as they do. 



Second, it has a low melting point. It melts imme- 

 diately if you hold a match to it; try this and see. 

 Consequently, long before the fuse wire becomes red 



