Lessons in practical electricity; principles, experiments, and arithmetical problems, an elementary text-book . t to flow throughthe lamp, which glowsbrighter for a momentand then becomes dimas the current attainsits steady value. Onbreaking the circuitthe lamp again glowsvery brilliantly sinceit is in circuit with theelectromagnet. The energy stored in the magnetic field is thus con-verted into a momentary direct current of a high E. M. F. and lightsthe lamp. The fields of a dynamo should not be broken when fullyexcited since the E. M. F. may become so high as to cause a punctureof the insula
Lessons in practical electricity; principles, experiments, and arithmetical problems, an elementary text-book . t to flow throughthe lamp, which glowsbrighter for a momentand then becomes dimas the current attainsits steady value. Onbreaking the circuitthe lamp again glowsvery brilliantly sinceit is in circuit with theelectromagnet. The energy stored in the magnetic field is thus con-verted into a momentary direct current of a high E. M. F. and lightsthe lamp. The fields of a dynamo should not be broken when fullyexcited since the E. M. F. may become so high as to cause a punctureof the insulation at two or more points, and thus complete the circuitthrough the iron core, and cause a ground, ^ 312. The self-inductionat break of such a circuit is termed the field discharge. 296. Inductance.—The cause of the self-induction is dueto the property possessed by the wire or coil called inductance,just as the resistance of a wire is that property of it whichopposes the flow of a current through it. A coil or wire,therefore, possesses inductance whether current is passing \ Switch Battery,•Hell*: wmm. l—illinium— Fig. 279. -Experiments Illustrating the ExtraCurrent of Self-induction. ELECTR OMA GNETIC IND UCTIOK 311 through it or not. The amount of inductance offered by acoil depends on the number of turns of wire in the coil andon the magnetic conductivity of the medium surrounding coil of 50 turns wound around an iron core has a verymuch higher inductance than a coil of 50 turns without aniron core. A coil of 15 turns wound on an iron core has lessinductance than one of 60 turns wound on a similar inductance of a coil or circuit is measured by the E. induced in it when the inducing current varies at anygiven rate. The unit of inductance is called the henry (thesymbol for which is L), and is the induction producedin any circuit when the induced E. M. F. is one volt, andthe current through the circuit varies at the rate of one am-pere per second. Th
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