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MANAGERS AND PROJECTIONISTS 421
to enable the projectionist to alter the position of the dial switch at will from working position beside either projector.
If conditions make it necessary to locate the rheostats inside the projection room then they should preferably be located near the ceiling, and if possible under a vent leading directly to the open air.
If it is not practical to have such a vent from the rheostats to the open air, then a hood should be provided, with a pipe leading therefrom directly into the projection room vent flue, so that the heat generated by the rheostats will be carried away.
Under no circumstances should rheostats be located on or near the floor of a projection room. Such location will not only breed discomfort in warm weather but will be decidedly dangerous, because of possible contact with film or other inflammable material.
EXAMINING CONNECTIONS.— It is important that the binding posts of the rheostat be frequently and carefully examined. Metal oxidizes under the action of heat, and if the rheostat terminals be left too long without attention, a thin scale is apt to form between the wire or the lug and the metal of the binding post. This scale may be quite visible, or it may be practically invisible. In either event it offers very high resistance.
It is an excellent plan to loosen the terminal connections of the rheostat at stated intervals (the length of same to be dependent upon the number of hours per day they are in use) and clean them thoroughly, either by the use of sandpaper, emery cloth or by scraping. This is particularly important if the rheostat be working at or above capacity. Where rheostats are used several hours every day, once a week is none too often to do this.
ADDING EXTRA RESISTANCE.— Should your rheostat deliver too much current when all the resistance is "in," or if from any other cause you should desire to increase its resistance, you may do so by mounting one or more extra rheostat coils as per A, Fig. 133. It is also possible to introduce extra resistance in this way by making up some coils of No. 8 soft iron wire (or larger size if it be a rheostat of larger amperage capacity), though this is not recommended, since iron wire has a very high temperature coefficient.
The extra coils may be mounted on porcelain insulators,