[Electric engineering.] . hows the lamps connected to an ordinary 110-voltdirect-current system. By using lamps with a slightly dif-ferent mechanism, they may be operated from the secondaryof a transformer, as shown in Fig. 22. When arc lamps areoperated from constant-potential direct-current mains, it isnecessary to connect a resistance r, Fig. 20, in series withthe arc. This is necessary for two reasons. In the firstplace, the lamps will not regulate well without it, and inthe second place, the voltages used on constant-potential § 18 ELECTRIC LIGHTING. 33 circuits are usually considerably h
[Electric engineering.] . hows the lamps connected to an ordinary 110-voltdirect-current system. By using lamps with a slightly dif-ferent mechanism, they may be operated from the secondaryof a transformer, as shown in Fig. 22. When arc lamps areoperated from constant-potential direct-current mains, it isnecessary to connect a resistance r, Fig. 20, in series withthe arc. This is necessary for two reasons. In the firstplace, the lamps will not regulate well without it, and inthe second place, the voltages used on constant-potential § 18 ELECTRIC LIGHTING. 33 circuits are usually considerably higher than the voltagerequired by a single arc lamp, so that the excess voltagemust be taken up in a resistance. If an arc lamp were connected directly to constant-poten-tial mains, without the intervention of any resistance, itsaction would be unstable. If the current flowing throughan arc increases, the resistance of the arc decreases, becausethe increased current causes the cross-section of the arc to Wlfc/fj \ s y rf. Fig. 21. increase. On the other hand, if the current decreases, theresistance of the arc increases. The consequence is that ifthe constant voltage of the mains was just equal to thatrequired by the arc and if the current through the arcshould, for any reason, decrease a little, the resistanceoffered by the arc would at once increase, thus causing afurther decrease of current and increase of resistance, withthe result that the arc would go out. On the other hand, anincrease of current would result in a decrease of resistance. 34 ELECTRIC LIGHTING. 18 and this would cause a still further increase of operation of the lamp would therefore be unstable,and would fail to maintain a constant arc for any lengthof time. Now, if a line voltage somewhat higher than that requiredby the lamp is used and sufficient resistance inserted to give
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