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the lower edge of the positive crater as shown in Fig. 3 -C, it would not have as much tendency to keep the hot gases in the crater and would result in a lower candle power. It was found in the case of the 16 mm. carbons in the current range of 140-150 amperes, which is ordinarily used, that the best arc voltage was 73-83 volts. Below this voltage the negative was so close to the positive that the negative flame appeared to impinge on the hot gases in the positive crater with such force as to actually drive them out with a consequent unsteadiness and loss of light. Above this voltage the negative was so far away from the positive that the negative flame apparently lacked the necessary force to confine the gases in the positive crater and caused a loss of light from the crater area. With lower currents, lower voltages can be used.
The 13.6 mm. carbons in the current range of 110-125 amperes operated best at approximately 67-73 volts and the 8 mm. carbons in the current range of 60-70 amperes operated best at 48-55 volts. In general, if lower currents are used, the volt
Nationai High Intensity Carbons
HORIZONTAL ANGLE FROM AXIS OF POSITIVE CARBON
Figure Four
age should be correspondingly decreased. The effect of lower and higher voltages with the 9 mm. and 13.6 mm. carbons is the same as with the 16 mm. carbons although to a somewhat smaller degree.
The angular distributions of candle power from the positive craters of 9, 13.6, and 16 mm. carbons in the horizontal plane in a total angle of 80 degrees are given in Fig. 4 for a number of different currents. The candle power is slightly lower directly in front of the crater than at 10 to 20 degrees on either side. The candle power holds up remarkably well to the 40-degree limit measured and is only 10 to 17 per cent lower at 40 degrees than at the center. This accounts for the decided increase in the useful light from the high intensity arc when a mirror or condensers of large effective angle10 are substituted for the old style condensing lenses of small effective angle.
The light distribution is approximately the same for the different sizes of carbons and the different current values investigated as is clearly shown in Fig. 4.
The areas of the crater openings of the different size carbons at the various currents are given in Fig. 5.
— r — i — i — i — i —
National High Intensity
Carbons
CRATEJ
7 OPENING VS. 1
^6M
A
UKkLN
1
1.6 MM.
9MM.
50 60 TO 60 90 100 1IO 120 130 VOO ISO 160
CURRENT
Figure Five
The cross-sectional areas of the 9 mm., 13.6 mm., and 16 mm. high intensity carbons are 64 sq. mm., 145 sq. mm., and 201 sq. mm., respectively. It is obvious from the curves that the crater openings for even the higher currents are much less than the original carbon cross-section.
Eleven