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THE LIGHT SOURCE 381
(79) It is wise to have spare positive carbon contacts in stock. Not only is it insurance against trouble, but also it enables the projectionist to install a clean set immediately before the day's run, cleaning the old ones at his leisure for use next day.
Positive Carbon Rotation and Feed Unit
(80) This part, shown in full detail in Fig. 152, serves to rotate and to feed the positive carbon forward as fast as it is consumed. The entire part rotates, carying the carbon with it, while at the same time toothed wheels A-A, driven by worm gears C-C, feed the carbon forward as it is burned away at the arc. Wheels A-A are held against the carbon by the pressure exerted by spring C as shown in Fig. 151. The tension of spring C may be altered at will by means of nut I, shown in the same illustration.
(81) The entire unit illustrated in Fig. 152 operates in high temperature and so its working parts must be kept well lubricated. We have always recommended a mixture of powdered graphite and kerosene but some lamp manufacturers object, claiming that the graphite binds the parts. That may be true of graphite grease but a well powdered, pure graphite will not bind up the parts and is one of the best high temperature lubricants known to science. A high grade automobile oil is also good. Whatever you use it is important to keep the parts well lubricated. Graphite, if used, must be in powdered form, mixed with kerosene. The kerosene will burn away at once leaving the part lubricated with dry graphite.
When Positive Carbon Fails to Feed
(82) The trouble usually may be traced to rough, pitted current-carrying contacts. Sometimes — but seldom— it is due to faults in the feeding mechanism. The parts must contact the carbon with sufficient pressure to insure its movement. The feeding toothed rollers are held against the carbon by coil spring C as in Fig. 151, the tension being adjustable by nut T.