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368 RICHARDSON'S BLUEBOOK OF PROJECTION
(59) In case the positive carbon ceases to rotate and feed forward, ease up on the contact pressure and try to finish the run. Then carefully examine the contacts for roughness. If they are rough remedy that at the earliest moment or the contact blocks will be ruined quickly.
(60) To make a quick test for roughness, place the lamp in re-trim position, release the feed mechanism and shove the positive carbon backward and forward several times through the current-carrying contact. With the front end of the carbon extending out in normal operating position, and with the feeding mechanism released, you should be able to move the carbon backward and forward easily. If it binds more than it should, then one of two things or both, is the trouble. Either the pressure of the contact blocks is too heavy or the contact surface is roughened. Smooth the contacts and (or) adjust the contact pressure.
High Intensity Carbon Contact Pressure
Too little contact pressure causes arcing and quickly roughened, burned contact surfaces. (61) Too much pressure places undue strain on the feeding mechanism and may cause the carbon to stick — to fail to rotate and feed. (62) There must be good electrical contact between the rotating carbon and its contacts or trouble will result.
(63) Always have spare feeding mechanism parts in stock, ready for use. Ordinarily the controlling spring and parts that contact the carbon are sufficient to have onhand for an emergency.
The Flame Shield
(64) All high intensity lamps have a flame or heat shield plate mounted between the arc and the positive current— carrying contact as a protection against the terrific heat of the arc. In some lamps the hole in thisplate through which the carbon passes is bushed with a separate metal part. (65) Neither the hole in the plate or the bushing must be allowed to fill up with carbon dust or oxide, until it reaches the carbon. (66) Carefully