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National Projector Carbons
carbons are clamped near the holder end and neither is rotated. It is therefore necessary to increase the conductance of the electrode by coating it with metal, which in this case is copper. The copper does not enter the arc stream, its main function being to furnish a low-resistance path for the current from the carbon holder to a point near the arc. It also protects the carbon from oxidation. Figures 60 to 65 illustrate the performance of the A.C. high-intensity arc under different operating conditions. These illustrations are all traced from actual arc images and show the true relationship between the different elements of the arc.
If the current is too great, the copper will melt to a considerable distance from the arc, as shown in Figure 60. The arc then becomes unsteady and is apt to blow out. Also, the arc voltage and consumption of the carbons are increased to such an extent that they may be outside the range of control of the arc-feeding mechanism. If, on the other hand, the current is too low, the copper will not melt away as far from the arc, the light will be very much reduced, and the current and voltage will not be constant. This condition results in an unsteady arc, which "flops" from the top to the bottom of the carbon, as illustrated in Figure 61. If the recommended current and voltage limits are observed neither of these undesirable conditions will be encountered.
Figure 60 8 mm A.c. High Intensity Carbons, Overloaded:
90 Amperes, 35 Volts
76