[Electric engineering.] . ght at different angles above and belowthe horizontal line passing through the arc that is locatedat a. The distance from a, measured along the radius atany given angle, is proportional to the candlepower of thelamp when viewed from that position. For example, thelight reaches its greatest intensity at a point about 45° belowthe horizontal and then rapidly diminishes on both sides ofthis point. Directly above or below the arc there is, ofcourse, little or no light, as the arc is obscured by the frameof the lamp and the carbons themselves. The open arc §18 ELECTRIC LIG
[Electric engineering.] . ght at different angles above and belowthe horizontal line passing through the arc that is locatedat a. The distance from a, measured along the radius atany given angle, is proportional to the candlepower of thelamp when viewed from that position. For example, thelight reaches its greatest intensity at a point about 45° belowthe horizontal and then rapidly diminishes on both sides ofthis point. Directly above or below the arc there is, ofcourse, little or no light, as the arc is obscured by the frameof the lamp and the carbons themselves. The open arc §18 ELECTRIC LIGHTING. 17 throws out comparatively little light in the horizontal direc-tion, and the quantity of light thrown upwards is is thus seen that the plain open-arc lamp using a directcurrent as it stands, without any reflector and with simplya clear-glass globe, gives a good distribution of light forstreet lighting because, on account of the formation of thecrater in the upper carbon, it throws the bulk of its light. SO 30° Fig. 13. downwards at an angle of about 45°, where it is most is one of the reasons why the direct-current open-arclamp has proved so successful for street lighting. If thedeep shadows directly under the lamp are objectionable,they may be softened by using a clear globe with the lowerhalf ground. 21. Open-Arc Alternating- Cur rent Lamps. — Thedistribution from an alternating-current open-arc lamp isnot of much practical importance because these lamps arenow seldom used. It is, however, instructive to compare itwith Fig. 13. Fig. 14 shows the general distribution froman alternating-current open arc, as determined by Uppen-born. It will be noticed that a great deal of the light is 18 ELECTRIC LIGHTING. §18 thrown above the horizontal. This is because the two carbon points are alternatively positive and negative, so that both become heatedto nearly an equal a lamp, to be effec-tive for street lighting,should be provided with a
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