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THE LIGHT SOURi 369
examine the hole before starting the first show each day. Whenever necessary remove the plate or bushing and clean out the hole. (67) Never attempt to operate a high intensity lamp without a flame shield. If a bushing is provided do not run without it in place. It is possible to do so, but the positive carbon will taper back too much and the contact blocks will soon be injured. Keep spare bushings on hand if your flame shields are equipped with them.
The Negative Head
(68) The negative head carries the negative carbon and its feeding mechanism. (69) It must be adjusted so that when the negative carbon tip is raised into contact with the positive it will be exactly central — sidewise in relation to the positive — it must be able to maintain this position until the negative carbon is consumed.
(70) In all lamps the negative carbon is clamped and has its main current-carrying contact several inches below its upper end, but just above this is a sliding loose contact through which the negative carbon receives a portion of its current. (71) The projectionist should see to it that the carbon bears firmly upon the metal of this groove, and that it lies central therein. This last is quite important. (72) If it does not, make whatever adjustment is necessary to line up the carbon properly. The method of adjustment differs with different makes of lam[).
(73) The main points to watch with relation to the negative head are keep the carbon in exact alignment, to keep moving parts (feeding mechanism) well lubricated, and to keep carbon contact clean and tight.
(74) The selection of proper carbon trims, burning them at the most efficient amperage for the trim and maintaining the exact correct position of the negative and positive carbon tips with relation to each other are all vital points, particularly with high intensity arcs.
(75) The high intensity carbon is very critical with regard to current flow, burning below the rated capacity results in a heavy loss in light production and very in