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MANAGERS AND PROJECTIONISTS 785
We have tried to make this very clear because it is essential that you understand the action of the carriage in order that you may understand the other details.
The positive carbon is rotated semi-continuously by gear 5-41, which is itself driven by a small gear immediately under it, 5-50, Fig. 303. This latter gear is carried along rod 5-31 as the carbon carriage moves forward, but it makes positive driving contact therewith by means of a slot in the shaft into which a key in the hole of the gear fits ; shaft 5-31 being rotated semi-continuously by means of ratchet 5-121, Fig. 304. The reason we say the rotation is semi-continuously is that while the action is to all intents and purposes continuous, still it is accomplished by means of pawl 518, Fig. 304, acting on its ratchet, hence it is not a strictly continuous movement. The feeding forward of the positive carbon is by means of a coarse threaded screw 5-35, which moves the whole positive carbon carriage forward, or pulls it back when necessary. The feeding of the positive carbon is not continuous, but is automatic and intermittent. The feed is designed to maintain the crater in a position, the possible variation from which is fixed within certain very definite and very small limits.
THIRD ELECTRODE.— The position of the positive crater and the feeding of the positive carbon to the arc is accomplished by a "third electrode" system, the main element of which is numbered 5-15, Fig. 303. The third electrode, 5-15, is a ribbed casting of a heat resisting metallic alloy. It is mounted just over the front end of the positive carbon, and is insulated from both positive and negative parts of the lamp., This later is accomplished by means of bracket 5-8, Fig. 303, which is insulated by material 5-11, Fig. 303.
HOW IT WORKS.— By means of screw 5-13, Fig. 303, the third electrode may be shoved ahead or retarded, and it is the position of the front tip of the third electrode with relation to the tip or end of the positive carbon which determines the position of the crater, or in other words, its distance from the face of part 5-3, Fig. 303. If part 5-15 be moved further out, then the crater will form further out, or vice versa. Part 5-15 should be so adjusted that the crater will form about 3/4 of an inch from the face of part 5-3, Fig. 303.
CAUTION.— After making an adjustment of the position of part 5-15 always tighten lock nut 5-14.
From part 5-15 an asbestos covered wire extends back to terminal 5-19, which connects with the automatic unit control, the operation of which will be described further along.