A laboratory manual of electrotherapeutics . two windings of wire. The inner has eightlayers of fine wire and the outer two layers of coarse the coarse wire windings are connected in series insuch a way that the magnetic polarity produced shall bealternately north seeking and south seeking, when a directcurrent is sent through this circuit. The armature is composed of several discs of sheetiron firmly fastened together, having slots and projectionsas will be seen from the figure. This armature is justlarge enough to rotate freely within the concentricallyarranged pole pieces. Bearing
A laboratory manual of electrotherapeutics . two windings of wire. The inner has eightlayers of fine wire and the outer two layers of coarse the coarse wire windings are connected in series insuch a way that the magnetic polarity produced shall bealternately north seeking and south seeking, when a directcurrent is sent through this circuit. The armature is composed of several discs of sheetiron firmly fastened together, having slots and projectionsas will be seen from the figure. This armature is justlarge enough to rotate freely within the concentricallyarranged pole pieces. Bearing in mind the relation of the OF ELECTROTHERAPEUTICS. secondary coil to the pole, it is evident that anythingcausing an increase or decrease of magnetic lines of forcethrough the pole, /. e., any variation of the magnetic flux,will induce a current of electricity in the secondary. TheE. M. F. of this induced current will depend upon thenumber of turns of wire and the rate of increase ordecrease of the magnetic flux. In this machine the direct. Fig. 42. current passing through the coarse wire windings consti-tutes the magneto-motive force; this remaining constant,the magnetic flux will vary inversely with the reluctance,i. e., the magnetic resistance. The law of the magnetic circuit may be stated as follows: magneto-motive force magnetic flux = -.— ° reluctance. The reluctance of air being very great as comparedwith that of iron it follows that the magnetic flux in any pole -I02 LABORATORY MANUAL will be suddenly increased when the air-gap or slot oppo-site that pole is displaced by the iron projection of therotating armature, thus inducing a current in one directionand the next instant when the wire projection is passingand the next slot is coming opposite the pole there will bea decrease of magnetic flux and a corresponding currentin the opposite direction. The slots and projections of the armature are soproportioned that the graphic representation of thesealternations of the
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