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The lamp in the average silent projector is usually one of 300 watts, though many are made to take lamps of 500 watts, whilst in sound-machines 750-watt lamps are in general use. Many of these lamps are rated for 115 volts, and a resistance or transformer is now a usual part of the equipment. The lower voltage lamps have three main advantages — less chance of the filaments fusing, stronger wires not likely to sag, and a greater proportion of light to heat.
Considerable care must be taken if the lamp is to last its full rated life, usually 30 to 50 showing hours. Avoid switching it on and off more than is necessary during a show, never turn the projector on its side while the lamp is still hot, and always avoid jerking or jarring the machine.
Behind the lamp is the reflector, and care must be taken to see that this is correctly aligned if of the adjustable type. To do this, switch on the lamp, remove the projection lens and hold a piece of white paper in front of the lens mount — you should then be able to see the shadows of the filaments upon the paper. The reflector will show a second lot of filaments, and their reflection on the paper should be aligned exactly between the rows of the others. If you find the reflections are out of place after changing the lamp then you must adjust the lamp, and not the reflector.
Many of these reflectors are made of glass, and as the lamps sometimes blister before burning out, great care must be taken not to allow a blister to touch the reflector as the latter may easily crack. It is quite in order to use a blistered lamp, but it must be removed before the blister gets too large.
In some cases a blistered lamp may have to be broken before it can be removed from the lamphouse — don't let this happen to you. Always change the lamp as soon as the blister necessitates a change.
Lamps sometimes blacken, and those giving poor light should be changed at once. Never step-up the voltage instead.
When changing a lamp always carry out the operation as quickly as possible, for the metal parts act as a cooling-flange when exposed to the air and soon become too hot to touch. Never change a lamp without first switching off the electricity supply.
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