c 



FUSES 367 



out around the wire, through the bell, or other instru- 

 ment, and back to its starting point. Let us, for example, 

 take the lighting system in our home. Current is brought 

 into the house by one wire, and returns to its source by 

 another. If in the 

 diagram, A and B 

 connect our house 

 circuit with the city 

 supply, when the 

 switch is closed we 



are able tO light any Diagram of lighting system. 



one or all of the lights L, L', and L" . These lamps have a 

 high resistance to electricity, and allow not more than one 

 to three amperes of current to flow through the circuit. 

 Now suppose the bare ends of the wires C and D, which 

 have low resistance, are brought together. This " short 

 circuit " would reduce the resistance several hundred times, 

 and the current would be increased enormously. 



There is a real danger from short circuits. Not only 

 may the insulation on wires be burned off, but the build- 

 ing may be set on fire. If any part of the wire AC is con- 

 nected metal to metal with any part of BD, or if the two 

 separated metal parts of the lamp circuit be joined by a 

 piece of metal, a short circuit results, with its great in- 

 crease of heat and corresponding danger from fire. When 

 the dry cell is " short circuited " there is much less danger 

 from fire than in the case of the electric light circuit, be- 

 cause the cell has a very small capacity. Yet this cell 

 will heat a short length of small iron or german silver 

 wire red hot, and so might be a source of danger. 



Fuses. If we were to take a number of different metals 

 and expose them to heat, we would find that some of them 



