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432 HANDBOOK OF PROJECTION FOR
obtained by figuring rheostat resistance will in the very nature of things be only roughly approximate.
As a matter of fact a wire coil rheostat rated at a given amperage, and which delivers that amperage when it is new, will not do so after it has been used for a time. The resistance of the wire coil rises gradually for a time, after which it remains practically stationary until the coils finally give out entirely. When the resistance reaches its highest point it will usually be found that the actual delivery of the rheostat will be from 5 to 10 per cent, less than the rated delivery.
This latter may or may not apply to any considerable extent to cast iron grids. It is claimed that the resistance of cast iron remains practically constant. We have been unable to secure reliable data substantiating the claim.
ADJUSTABLE RHEOSTAT AS IT IS.— In Fig. 132 we have the photographic representation of the single grid of a grid rheostat, and the photographic representation of the single coil of a wire coil rheostat. At the top in Fig. 139 we have a side and bottom view of a grid rheostat, the grids of which are numbered consecutively from 13 to 26. This particular rheostat is out of date, but it serves very well to illustrate what we have in mind. It will be observed that between each grid at the top is a spacing washer. These washers are alternately lettered X and O. The X washers are current carrying (metal) and form an electrical connection between two adjoining grids. The spacing washers, O, are of insulating material. They insulate adjoining grids between which they are placed from each other at that end. The nut on the end of the long bolt which clamp the whole thing together is numbered 4. This bolt is insulated from the metal of the grids. At the bottom is a similar bolt, also numbered 4 and also insulated from the grids. At the bottom of the grids are similar spacing washers, but if there is a metal spacing (X) washer (current carrying) between two grids at the top there will be an O (insulating) spacing washer between the same two grids at the bottom.
Examining this arrangement in detail we see that, starting at the bottom of the left hand outside grid the current passes up through the "wires" of the grid (a grid is, as you will see in Fig. 132, really nothing but a cast iron wire) to the top, through washer X into the second grid, dowii through this grid, through the current carrying washer between it and the third grid, up the third grid, across the