ELECTRIC LIGHT 



1991 



ELECTRIC LIGHT 



Lighting Circuits. Incandescent lamps are 

 usually connected in multiple, with such an 

 arrangement that each lamp may be turned 

 on and off independently of the others. Low 

 voltage lamps may be used on high voltage 

 circuits by connecting them in series. For ex- 

 ample: Suppose the voltage of a trolley line 

 is 550 volts. The same lamps may be used in 

 the street car as on a 110-volt circuit, but in 

 the street car it is necessary to connect five 

 110-volt lamps in series. When this group of 

 five lamps is connected to n 550-volt circuit 

 there is a pressure of 110 volts at the terminals 

 of each lamp. Low-voltage lamps, both carbon 

 and tungsten, are made for use with batteries. 

 The most extensive use of such lamps is for 

 automobile lighting. 



A common form of wiring lighting circuits 

 is the three-wire system. Two of these wires, 

 commonly called the live wires, form a 220- 

 volt circuit. The third wire is neutral. The 

 difference of potential between either of the 

 live wires and the neutral wire is 110 volts. 



The Nernst Lamp. This lamp depends on 

 the strange fact that many insulating sub- 

 stances become conductors of electricity at 

 high temperatures. Even glass, which is one of 

 the best insulators, when heated to a red heat 

 becomes a conductor, and a current may then 

 be passed through it sufficiently strong to 

 cause it to incandesce, that is, to become white 

 hot; and the glass may even be melted by the 

 heat developed in it by the electric current. 



The essential part of the Nernst lamp is the 

 glower, a small rod of rare earths mixed with 

 a binding material. This rod is an insulator at 

 ordinary temperatures; therefore it must be 

 heated to make it a conductor. The heater is 

 a coil of platinum wire mounted close to the 

 glower. When the current is turned on, the 

 platinum wire quickly becomes white hot and 

 heats the glower. As soon as the glower is 

 heated sufficiently to conduct the current, the 

 current passes through it and it instantly glows 

 with an intense white light. The platinum coil 

 is then automatically cut out of the circuit. 

 The glower lights at a heat of about 1,740 

 Fahrenheit. 



Mercury Vapor Lamp. The mercury vapor 

 lamp gives out a bluish-white light containing 

 almost no red rays. This light is good for 

 photographic purposes, eince the blue rays 

 which it contains readily act on the photo- 

 graphic plate. In such a light a red rose ap- 

 pears almost black. The face of a person ap- 

 pears almost ghastly pale, the lips purple as 





if from asphyxiation. This lamp is a 

 vacuum tube, usually three feet or more in 

 length. The cathode is mercury; the anode, 

 iron. To light the tube it must be placed in a 

 horizontal position until the mercury extends 

 the entire length of the tube and completes 

 the electric circuit; then the tube is tilted BO 

 that the mercury drops back to one end. The 

 electric current vaporizes a portion of the mer- 

 cury and continues to flow, the mercury vapor 

 being a good electrical conductor. The cur- 

 rent causes the mercury vapor which now fills 

 the tube to glow with a bright bluish-white light. 



Lighting Trains and Automobiles. For light- 

 ing trains on a steam 

 railroad, a combina- 

 tion of dynamo and 

 storage battery i & 

 used. A dynamo 

 geared to an axle of 

 the car furnishes cur- 

 rent for the lights 

 when the train is run- 

 ning. When the train 

 stops a storage bat- 

 tery is automatically 

 switched into the cir- 

 cuit. 



For electric lighting 

 of automobiles a sim- 

 ilar system on a small 

 scale is used. The 

 dynamo not only fur- 

 nishes current for the 

 lamps when the auto- 

 mobile is running, but 

 charges the storage 

 battery. 



The Arc Light. 

 When two rods of car- 

 bon are made part of 

 an electric circuit so 

 that a current flows 

 through them, and I 

 the carbons are then 

 separated by a short 

 distance, the current 

 continues to flow; but 

 to flow from one car- 

 bon to the other it 

 must cross the gap 

 between them. This 

 gap, if in air, offers 

 very great resistance 

 to the current. On account of this resistance 

 intense heat is produced so intense that it 



ARC LIGHT 

 Enclosed. 



