[Electric engineering.] . nstructive 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 areflector to throw the lightdown where it is curves shown inFigs. 13 and 14 representaverage distributions
[Electric engineering.] . nstructive 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 areflector to throw the lightdown where it is curves shown inFigs. 13 and 14 representaverage distributions. Itmust be remembered thatthe arc always shiftsaround more or less, andhence the shape of thedistribution is constantlychanging and makes themeasurement of the lightgiven out in the variousdirections an exceeding-ly difficult matter. Thecurves will, however, en-able the student to notethe marked difference infig. 14. the light distribution of the alternating-current open arc as compared with the direct-current open 22. Enclosed-Arc Direct-Current Damps. — Therehas been a great deal of discussion regarding the light-giving properties and efficiency of the enclosed arc ascompared with the open arc. The data here given isabstracted from a report of a committee of the NationalElectric Light Association on tests made by Prof. C. , and is probably as unbiased as any 15 shows the average of curves from direct-cur-rent 110-volt enclosed-arc lamps used on constant-potential § 13 ELECTRIC LIGHTING. 19 circuits. Curve A shows the distribution when the lamp isprovided with an opalescent inner globe only; there is nolarger outer globe. The student should compare this curvewith that shown in Fig. 13 for the open arc. It will benoticed that with the enclosed arc, the light is of fairly largeintensity through a considerable angle below the this case, the maximum value is approximately 3G0 candle-power and occurs about30° below the horizontal.
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