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When the groove has been filled the soft-iron pieces are calked or spread so as to hold the blades firmly in place. This upsetting is done by a tool which shears the tail of the comma at the proper width between the blades.
Indeed, he grew so interested in Swain's exposition of deviation and variation and magnetic attraction and the various devices employed to counteract these influences, the Flinders bars, the soft-iron spheres, and the system of adjustable magnets located in the pedestal of the binnacle, that he had to be reminded by a mild exhibition of sisterly temper that she hadn't summoned him to the bridge for his private edification.
In an electromagnet the magnetism exists only as long as current is flowing through the wire, and the polarity of the soft-iron bar is determined by the DIRECTION of flow of current around it for the time being. If the direction is reversed, the polarity will also be reversed.
He had picked up the little ball. "This ball," he said as he examined it, "is not really of ivory, but of a composition that looks like ivory, coating a hollow, soft-iron ball inside. Soft iron is attracted by an electro-magnet. Whichever set of magnets is energized attracts the ball and by this simple method it is in the power of the operator to let the ball go to red or black as he may wish.
The exciting current was at first obtained from a small permanent magneto machine; but it was afterward found that the machine could be made self-exciting. Soft-iron electro-magnets, after being once magnetized, remain slightly magnetic. This will produce a weak current in the revolving armature which is turned into the magnet coils.
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