564 CANDLE-POWER OF ARC LAMPS [Cn. XIII 



o Direct current (D. C.). 



x Alternating Current (A. C.). 



The direction of a given point on the curve represents the direction in which 

 the light intensity was measured. The distance of the point from the center 

 of the figure represents the intensity in the given direction. For example, 15 

 above the horizontal the direct current arc has 2,900 candle-power while the 

 alternating current arc has 850 candle-power. 



The numbers around the outside represent the angle in degrees while those 

 on the radius represent candle-power. 



carbons or the angle of the craters on the carbons causes a great 

 change in the distribution of light. 



770. Right-angle electrodes. If the right-angle arc is used, 

 take care to hold the crater in the best position, i. e., facing the 

 condenser, otherwise a poor light will result. Fig. 294-295 show 

 about the best position which can be maintained. The distribu- 

 tion from this arc with direct current is shown in fig. 305. The 

 distribution of light with an alternating current right-angle arc is 

 shown in fig. 306. 



771. Converging electrodes. The distribution of light with 

 converging carbons (55) with alternating current is shown in fig. 

 306. 



CANDLE-POWER OF ARC LAMPS 



773. Intrinsic brilliancy of the crater. Blondel found that 

 the intrinsic brilliancy of the positive crater of the carbon arc was 

 nearly constant, irrespective of the current, at about 158 candle- 

 power per square millimeter for solid carbons, and 130 candle-power 

 per square millimeter for cored carbons. This is equivalent to 

 97,000 candle-power per square inch for solid, and 84,000 candle- 

 power for cored carbons ( 7733). 



The increase in candle-power of the arc caused by an increase in 

 current is due, not to an increase in the brightness of the crater, 

 but to an increase in its area. This is illustrated in fig. 294, which 

 shows a photograph of a right-angle arc with 10 amperes and with 

 20 amperes direct current. The increase in the size of the crater 

 is apparent. 



As has been pointed out elsewhere (Ch. IX, XIV), with small 

 openings such as with microscopic objectives, when the crater 



