228 WATER, HEAT, AIR, AND LIGHT IN THE HOUSE 



258. Electric Lighting. The principle of the arc and 

 incandescent electric lamps has already been given (cf. 

 157). The filaments of modern incandescent lamps are 

 made of the rare elements tungsten and tantalum, as well 

 as of carbon. Tungsten and tantalum lamps cost more 

 than carbon lamps; but they use much less electric energy 

 to produce a given amount of light. 



It is very important to have a house properly " wired" ; 

 for if wires that are close together lose their insulation, 

 we have the accident known as ' ' crossed wires/ 7 or ' i short- 

 circuiting." If the wires were of large diameter, this 

 accident would not be particularly serious. But as copper 

 is expensive, the smallest possible wires are used, and these 

 are often unable to carry the "load" suddenly put upon 

 them. They become hot, and may set the house on fire. 

 To avoid the rise of temperature in a 

 circuit to the danger point, the electri- 

 cian places a "fuse box" (Fig. 214) 

 in the house. To "fuse" means to 

 "melt." 



The fuse box contains a number of wires 

 made out of a low-melting mixture (alloy) of 

 metals. One of these is placed in each circuit. 

 If for some reason the amount of current in a 

 circuit becomes too great, the melting of the 

 fuse breaks the circuit, and prevents a fire. 



The fuse box of a house usually contains, 

 also, the switch that controls all the electric circuits of the house. 

 When any repairing of wires or outlets is to be done, the house current 

 should be cut off by means of this switch. 



259. The Electric Meter. The work done by the elec- 

 tric current (cf. 25) is measured in watt-hours, or in 



USE METAL 



Fig. 214. 

 A Fuse Box. 



