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FOR MANAGERS AND OPERATORS 261
disconnected. Next touch one binding post and the outer casing or frame of the rheostat. If you get no ring then the rheostat may be considered in good order, except for one thing which cannot be located with a bell or test lamp, viz., two coils being sagged together so as to eliminate a part of the resistance without breaking the circuit.
The rheostat may be tested with a test lamp in a number of different ways. First, assuming the rheostat to rest upon a marble slab, or other insulating material, with the current on, touch your test lamp to the frame of the rheostat and to the wire of opposite polarity. If you get a spark, or light, the coils or grids are grounded to the frame, and the ground can be located as hereinafter described. Another way would be to disconnect the wire leading from the rheostat to the lamp from the rheostat binding post and, with the switch closed, touch the frame of the rheostat with one test lamp lead and the wire which has just been disconnected with the other, the arc lamp carbons being "frozen," i.e., in contact with each other. If you get a light or a spark there is a ground; if not, there is none. Still another way, again assuming the rheostat to rest on an insulating shelf, disconnect one of the wires from the rheostat binding post and, with the carbons of the lamp frozen and the switch closed, touch the disconnected wire end to the frame of the rheostat. If you get a spark there is a ground.
Now suppose you have applied one of these tests and find there is a ground in the rheostat, indicating that one of the coils is electrically connected with the frame. How are you going to locate the particular coil or grid at fault? This is a point which puzzles so many operators, yet it is as simple as a, b, c, when you come to examine it in the light of common sense. Close the switch, and, if you are using a test lamp, attach one test lamp lead to one of the rheostat binding posts. Now attach the other test lamp lead to the frame of the rheostat, and, beginning at the end farthest from the binding post the test lamp lead is attached to, disconnect the first coil, which we will assume to be coil A, Fig. 109. The light still burns. Disconnect coils B, C, and D in turn. The light still burns. Disconnect coil E and the light goes out, because you have removed the ground. You will, therefore, proceed to examine coil or grid E and locate the trouble, which may and probably will be due to a ground through the insulation of the connection at Z.
Where a rheostat consists of two blanks of coils or grids considerable labor can be saved by disconnecting one side