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,360 HANDBOOK OF PROJECTION FOR
First disconnect the wire leading from the rheostat to the arc lamp, leaving only the wire connected which leads from the source of electrical supply to the rheostat. Now, first having removed the casing of the rheostat, connect one of your test lamp wires to the frame of the rheostat and the other test lamp wire to a wire of opposite polarity. Assuming we have disconnected the wire from the left-hand binuing post, in Fig. 103, we will disconnect coil or grid A, and if the test lamp still burns, or if there is still a spark when its contact with the rheostat frame is made and broken, we know the trouble is not in A, since the ground still exists. We therefore connect BCD and E. When coil or grid E has been disconnected the test lamp goes out or the spark ceases, hence we know the trouble lies in that coil or grid. The trouble in coil or grid E may be due to direct connection with the frame caused by sagging, or it may be and probably is due to a fault in the insulation.
If a rheostat consists of two banks of coils or grids, considerable labor can be saved by disconnecting one bank from the other, and then testing each as a whole to find out which half the ground is in. It is then only necessary to disconnect the individual coils or grids of the defective side.
GROUNDING THE PROJECTOR.— It is always advisable that the projector lamp house, mechanism and frame be permanently grounded to the metal of the projection room, if any there be, and then the whole may or may not be thoroughly grounded permanently to a water pipe.
The reason for grounding the projector to the projection room metal work is that if the projector be insulated from the metal of the projection room and the lamp should become grounded to the metal of the lamp house it would charge the whole mechanism with voltage, and, should the projectionist in the act of putting a reel in the magazine touch the reel to the magazine and the metal of the projection room there would be a spark which might set fire to the film.
There is no real necessity for the grounding of the metal of the projection room as a whole. It may or may not be done, as best suits the idea of the individual.
EFFECT OF GROUNDING.— The effect of grounding the projector lamp is that current is wasted and the brilliancy of the light is itself likely to be affected, particularly if the ground be a heavy one, since a portion of the current is escaping through the shunt circuit produced by the ground,