ELECTRIC LIGHT 



1992 



ELECTRIC MACHINE 



vaporizes the carbon. This carbon vapor 

 crosses the gap in a bluish stream curved like 

 an arc of a circle, hence the term electric arc. 

 The stream of carbon vapor is curved by the 

 magnetic action of the current which it carries. 



The heat of the electric arc is so intense that 

 it is used for the electric furnace. If the arc 

 ia produced by a direct current, the carbon 

 vapor flows from the 

 positive to the negative 

 carbon, and the tip of 

 the positive carbon be- 

 comes hollow on ac- 

 count of the carbon 

 flowing from it. This 

 hollow space is called 

 the "crater," and at the 

 bottom of this crater is 

 the highest temperature 

 of the arc nearly 

 6,300 Fahrenheit. 

 When the arc is pro- 

 duced by an alternat- 

 , ing current no crater is 

 produced, for each car- 

 bon is alternately posi- 

 tive and negative. 



Every arc lamp is 

 provided with a mech- 

 anism which automati- 

 cally "feeds" the car- 

 bons as they burn out, 

 keeping them at a con- 

 stant distance from 

 each other. 



A searchlight is an 



ARC LIGHT 

 Showing parts. 



electric arc with a reflector for sending the 

 beam in any desired direction. For such a light 

 a focusing lamp is required. This lamp has a 

 mechanism which feeds the positive carbon 

 twice as fast as the negative carbon. This keeps 

 the arc in the same position, since the positive 

 carbon burns out twice as fast as the nega- 

 tive. Such lamps are used also for lighthouses, 

 theater illumination, scenic effects, stereopti- 

 cons, moving picture machines, photo-engrav- 

 ing, and as electric headlights in locomotives. 



The flame arc is produced by means of car- 

 bons having cores which are impregnated with 

 certain metallic salts which produce a yellow 

 flame as the arc burns. The flame arc gives 

 a more brilliant light, with greater penetrating 

 power, than the ordinary arc. 



The enclosed- arc is an arc almost completely 

 enclosed in a glass globe. In such a lamp the 

 carbons are consumed very slowly because of 



the small amount of air admitted. The light, 

 although less intense than would be yielded by 

 the same electric power in open air, is more 

 uniformly distributed. 



Electric current can be used to light cities 

 hundreds of miles from the source of the cur- 

 rent. The current generated at Niagara Falls 

 lights cities 250 miles away. The actual cost 

 of electric lighting to the householder is slightly 

 more than the cost of gas lighting for an equal 

 amount of light, but the electric light more 

 than saves its extra cost by its cleanliness. 

 Walls and furniture require less attention in 

 an electrically-lighted room than in one lighted 

 by gas. 



Dangers from electric wiring have been 

 greatly exaggerated. When the wires are run 

 in conduits, as they should be, there is abso- 

 lutely no danger. Fires may result under very 

 exceptional conditions when the wiring is "ex- 

 posed," that is, supported on porcelain knobs 

 instead of being run in metal conduits. The 

 conditions necessary to cause a fire are that the 

 insulation must be worn off both wires, and the 

 bare wires must come in contact with each 

 other and at the same time in contact with 

 dry wood or some other combustible material. 

 Even then the chances are that a fuse would be 

 blown in the "cut-out box" and a fire pre- 

 vented. E.E.B. 



The measurement of electrical energy used in 

 lighting is explained under ELECTRIC METER. 

 Consult Franklin's Electric Lighting, and Stand- 

 ard Handbook for Electrical Engineers; Bell's 

 Art of Illumination; Parker's American Hand- 

 book for Electrical Engineers. 



Related Subjects. The reader is referred to 

 the following titles in these volumes, in addition 

 to those named above : 

 Current Insulator 



Davy, Sir Humphry Tungsten 

 Electric Battery 



ELECTRIC MACHINE, a machine which 

 produces charges of positive and negative elec- 

 trification by friction or induction. 



The first electric machine was made by Otto 

 von Guericke of Magdeburg, about 1663. It 

 consisted of a globe of sulphur on an axis 

 which could be turned by hand. When the 

 dry hands were held against the rotating globe 

 electric charges were produced by the friction 

 (see ELECTRICITY). Revolving glass globes and 

 glass plates rubbed by leather pads were used 

 later. Only a small charge is produced by a 

 single rotation of the glass plate, but the 

 machine is so arranged that these charges 

 accumulate; after many rotations of the plate 



