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434 HANDBOOK OF PROJECTION FOR
the right to contact 2. The wire from contact 2 connecting to binding post B which, as you will see, operates to "cut out" the two left-hand grids, thus reducing the resistance of the rheostat as a whole. Contacts 3, 4 and 5 of the dial switch connect respectively to binding post C, D and E. The whole thing is diagrammatically represented in Fig. 131. In. order to disassemble a rheostat of this type one must
Figure 141.
first remove the outer casing, which is held to the frame by a number of screws, then the wires connecting to binding post A, B, C, D, E, F and G should be disconnected and a properly marked tag tied to each, so that no mistake will be made in reassembling. The dial switchboard may then be taken off by removing screws H-H. Following this the whole grid bank must be removed from the frame, and nuts 4-4 be removed, after which the grids may be disassembled, but great care should be taken not to injure the insulation.
The projectionist will do well before disassembling a grid rheostat to have at least one new insulating barrel for bolt 4, and some new insulating spacing washers.
The fixed resistance grid rheostat offers no different problem of construction, except that the dial switch and its connections are absent. The grids are insulated from each other in precisely the same way, binding post A-G being the only two such a rheostat will have.
At the bottom in Fig. 139 we have a view of the bottom of the rheostat shown above it. Such a rheostat may or may not have the sides of its individual grids supported by a stiffener, which is made of sheet steel lined with asbestos and is slipped on the side of the grid to strengthen the grid.
At Z in the lower half of Fig. 139 we see the spring contact of the dial switch. It is essential that spring Z make good