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CD., Old Type Low Intensity Lamps
CONDENSER LENS
Figure 24 Diagram of Old Type, Low Intensity, D.C. Projection Lamp
As pointed out in Chapter III, the positive carbon used in this type of lamp is always cored, that is, the center of the carbon is filled with a softer mix of carbon known as the core. While the brilliancy of that portion of the crater occupied by the core is a little lower than that of the surrounding solid carbon, this slight disadvantage is more than offset by the greater steadiness given to the arc. In fact, it would be impossible to produce a light of sufficient steadiness for satisfactory projection with solid positive carbons.
Since all of the light that is useful for projection in this type of lamp comes from the crater of the positive carbon, it is essential that the adjustment of the carbons, as they are consumed, be such as to maintain the positive crater in a fixed position where it can be accurately focused on the aperture plate. Likewise, it is essential that the arc stream should not wander from point to point, but should impinge upon the positive carbon in a steady position.
Prior to the advent of National Projector Carbons it was common practice to use a large diameter regular solid or cored projector carbon in the lower or negative holder. In some cases, a negative carbon slightly smaller in diameter than the positive carbon was used, whereas in others, the negative carbon was the same diameter as the positive. With the negative
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