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THE LIGHT SOURCE 383
The Flame Shield
(90) 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 this plate through which the carbon passes is bushed with a separate metal part. (91) 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. (92) Carefully examine the hole before starting the first show each day. Whenever necessary remove the plate or bushing and clean out the hole. (93) 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
(94) The negative head carries the negative carbon and its feeding mechanism. (95) It must be adjusted so that when the negative carbon tip is raised into contact with the positive it will be exactly central in lateral relation to the positive. It must be able to maintain this position until the negative carbon is consumed.
(96) 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. (97) 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. (98) If it does not, make whatever adjustment is necessary to line up the carbon properly. The method of adjustment differs with different makes of lamp.
(99) The main points to watch with relation to the negative head are to keep carbon in exact alignment, to