ELECTRIC LIGHTING 53 



devices. The common street open arc lamp may give as high 

 as 2000 candlepower of light. Lamps having still greater 

 power are sometimes used for special purposes. 



67. The Enclosed Arc Lamp. Some years ago it was dis- 

 covered that, if the carbons of an arc light were enclosed in a 

 small glass globe nearly air-tight, the light produced was 

 different from that produced by the open arc. For most 

 purposes the light from the enclosed arc is preferred to that 

 from the open arc. Since the small globe is almost air-tight 

 it is readily seen that the burning carbons will soon consume 

 all of the oxygen in the globe, and after that will merely glow 

 and not burn. The carbons on the enclosed arc lamp last 

 about ten times as long as those on the open arc. This means 

 a great saving of time in taking care of the lamps. During 

 recent years the enclosed arc lamp largely displaced the open 

 arc, not only for indoor lighting, but also for street lighting. 



At the present time, the incandescent tungsten lamp is 

 rapidly displacing the enclosed arc lamp both for indoor and 

 for street lighting. 



68. Electric Wiring. Electric wiring of buildings must be 

 carefully done by competent electricians. As we have already 

 seen, Art. 64, Exp. 23, electric wires become very hot if 

 they are too small to carry the amount of current sent over 

 them. It is also true that, if two wires carrying currents 

 come directly into contact with each other, a spark passes 

 between them. Occasionally fires are started by this "cross- 

 ing" of "live" wires. Most electric wires are of copper 

 and are covered with insulation. This insulation is a thick 

 covering of material through which very little electricity 

 passes. If the wires are not sufficiently large to carry the 

 current required they become so hot as to destroy this insula- 

 tion. Two of the wires upon which the insulation has been 

 destroyed may come in contact, producing a spark which may 

 set fire to the building. 



In the better modern buildings all electric wires are run in 

 CONDUITS. These conduits are simply iron tubes. They are 



