. American telephone practice . wo ends of 128 AMERICAN TELEPHONE PRACTICE. the U-shapecl permanent magnet are of the same polarity, itsmiddle being of the opposite polarity. The two coils are mountedon an iron cross-bar extending between the legs of this permanentmagnet. The poles of the electro-magnets therefore partake of thepolarity of the permanent magnet ends, while the armature, sup-ported from the center of the permanent magnet, becomes of theopposite polarity. The action of this ringer is usually misunder-stood at first sight, the natural supposition being that the limbs ofthe permane
. American telephone practice . wo ends of 128 AMERICAN TELEPHONE PRACTICE. the U-shapecl permanent magnet are of the same polarity, itsmiddle being of the opposite polarity. The two coils are mountedon an iron cross-bar extending between the legs of this permanentmagnet. The poles of the electro-magnets therefore partake of thepolarity of the permanent magnet ends, while the armature, sup-ported from the center of the permanent magnet, becomes of theopposite polarity. The action of this ringer is usually misunder-stood at first sight, the natural supposition being that the limbs ofthe permanent magnet are of opposite polarity. This ringer is veryefficient, and it is surprising that it should not have remained inservice, perhaps in a somewhat modified form. Besides being de-sirable in point of efficiency, it has the advantage of having itsworking parts partially enclosed by the rigid permanent magnet,which serves to protect them from mechanical injury. All of the generators so far described have been adapted to give. FIG. 103.—OLD WILLIAMS-ABBOTT RINGER. only alternating currents, the current consisting of a positive andnegative impulse for every complete revolution of the is sometimes desirable to make all of the impulses in the samedirection, as in certain classes of party line ringer, and for this pur-pose a form of commutator is sometimes applied to the end of thearmature shaft which will rectify current reversing every alternateimpulse. Such current is frequently spoken of as direct current,but more often as pulsating current, and a generator providedwith this device is termed a direct current or pulsating currentmagneto generator. In party line working it is often necessary to make ringers thatwill respond to only one direction of current; that is, to impulsesof one direction only, whether positive or negative. This is accom-plished by adding to the ordinary ringer a little coiled spring MAGNETO CALLING APPARATUS. 129 tending to hold the armature
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