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420 HANDBOOK OF PROJECTION FOR
leakage from the rheostats. In such cases the ground was of such high resistance that the leakage was not sufficient to either overheat the wires or blow the fuses, but it was of considerable amount, and the leakage was constant every moment the rheostat was in use.
WARNING. — Rheostats should never, under any conditions, be placed on an iron covered shelf, or on any other material connected with ground which will carry current in event of the resistance element becoming grounded to the frame of the rheostat. Always place rheostats ON NONINFLAMMABLE INSULATING MATERIAL.
COOLING THE RHEOSTAT.— One excellent way of dissipating the heat generated by the rheostat and blowing it out of the room is to set the rheostat in front of an opening, in some location where it will be safe to remove the outer casing. Remove the outer casing and set a small fan in front of the coils or grids in such way that the blast from the fan will blow through them and blow the heat out of the room This has the double advantage of getting rid of the objectionable heat and increasing the capacity of the rheostat. (Also see "New Rheostat/' Page 437.)
LOCATION OF RHEOSTATS.— Rheostats become hotsometimes very hot. It is therefore unsafe to locate them very close to any sort of inflammable material. Rheostats should never be located within less than one foot of any wall containing inflammable material unless a sheet of V$ inch asbestos be established between the rheostat and the wall, with at least a two inch air space between the asbestos and the wall. Rheostats should, of course, be thoroughly protected against any possible contact with inflammable substance of any kind.
As regards the location of rheostats in the individual projection room, so very much depends on the local condition that only general rules can be given, (a) Where it is practical it is always best to locate the rheostats outside the projection room. This is particularly true in warm climates. The projection room is likely to be more or less uncomfortably warm in summer, and if we add to the heat generated by the electric arcs and the natural heat of the weather the heat of anywhere from one to three rheostats, a very uncomfortable condition is likely to result. It is quite entirely possible to place an adjustable rheostat outside the projection room and so connect the lever of its dial switch by means of a series of levers and rods, or cords, as