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294 D. B. JOY AND A. C. DOWNES [j. s. M. P. B.
by the angle and relative position of the negative carbon with respect to the positive crater and by the voltage maintained across the arc. The angle is usually fixed by the construction of the lamp. The best results were obtained when the relative positions of the negative carbon and positive crater were such that the negative flame just brushed the lower edge of the positive crater as shown in Fig. 3-A. If the negative flame bathed the lower outside of the positive crater appreciably, as shown in Fig. 3-B, the candle power decreased probably because some of the current was taken on the outside of the positive crater thus lowering the current and energy concentration on the inside of the crater. If the edge of the negative flame were considerably ahead of 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
A B
FIG. 3. Effect of position of negative.
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 9 mm. carbons in the current range of 60-70 amperes operated best at 48—55 volts. In general, if lower currents are used, the voltage should be correspondingly decreased. The effect of lower and higher voltages with the