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THE LIGHT SOURCE 373
will function only when unnecessary variation in the arc gap occurs. It is entirely possible to so design an arc control that less than J/ -volt variation in arc voltage will cause it to function and feed the carbons forward. Controllers that will not do this should be refused.
Keep all electrical contacts clean and tight. Take loose lugs from the lamp binding posts and clean the contact surfaces with crocus cloth or 00 sandpaper (preferably the cloth) once each week in all-day theatres and once in two weeks in two-a-day theatres. Even if the metal looks clean take the precaution to polish the surfaces with the crocus cloth or lightly with the 00 sandpaper. A thin, almost invisible scale of oxide coating having high resistance power frequently forms, especially if the contact is not set up very tight.
(94) Examine all wires inside the lamphouse frequently. If they feel stiff and springy they are alright. If they do not, slit the insulation, pry the strands apart and examine them. If the interior of the wires is brown instead of copper colored, they are already offering too much resistance to the current flow and should be replaced immediately. Wires that are brown on the outside but still do not bend easily and are copper colored inside (asbestos-covered stranded wire) are still fit for use but should be examined frequently.
See to it that the backing of the collector-converging mirror is in good condition and that its face is perfectly clean and nowhere pitted. (95) Tt is an excellent plan t<ยป wash the mirror each morning while it is still cold. Use strong soap suds, rinse it off thoroughly and then polish. Re careful not to get the back of the mirror wet, as water damages some types of mirror backing.
(96) Mirrors may be cleaned with a fifty-fifty solution of water and pure grain alcohol. Never use wood alcohol or denatured alcohol because it will leave a thin coating on the glass.
Positioning the Mirror
(97) The light source must be at precisely the right distance from the mirror, and the mirror must be exactly