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42 RICHARDSON'S BLUEBOOK OF PROJECTION
type as compared with wire coil rheostats are outlined below :
Advantages
(1) Grids are not so likely to sag and thus short circuit some of their elements, or become grounded to the casing as are coils. (2) Grids last longer than coils under the srme conditions of service. (3) Grids are better able to withstand high temperatures than are coils. (4) Grids deteriorate very slowly.
Disadvantages
(1) i A grid rheostat is much heavier than wire coil one of equal capacity. (2) Grids m:iy be broken by a heavy jar. (3) Grids are more difficult to replace than are coils. (4) Temperature coefficient low and less fixed than that of the wire coil, hence grid somewhat less reliable as to stability in current delivery.
Inspection
(42) Resistances should be inspected carefully once each week and they should have a thorough examination on the first of each month. (43) As metal oxidizes under heat, wires should be removed from the binding posts each week and both wires and posts cleaned thoroughly with crocus cloth or 00 candpaper, preferably with sandpaper. They may look clean, but an almost invisible, high resistance scale forms nevertheless. Inspection once each week is based upon many hours of use each day. When there are only two shows a day examination once in two weeks is sufficient, but the thirty day inspection requirement holds good everywhere. This is especially important where rheostats are working at capacity or above.
Location of Rheostats
(44) Rheostats generate high temperatures. (45) Therefore they must not be located near inflammable material. (46) They should never be located in the main projection room in warm climates because of their heat-producing powers. They should have a vent outlet to open air.
(47) Under no circumstances should they be located at the floor level of a" projection room, because of possible contact with film and because such location would tend to add to the heat of the room.