Davis's manual of magnetism : including galvanism, magnetism, electro-magnetism, electro-dynamics, magneto-electricity, and thermo-electricity . a steel mag-net is placed in a proper po-sition near the revolving bar,^ ^ in either of the instruments whose motion depends on the earth^s influence, itrotates with much greater speed than by the actionof terrestrial magnetism alone. Its motion may bereversed, notwithstanding the opposing influence ofthe earth, by disposing the permanent magnet in asuitable manner. 347. Electro-Magnet revolving within a Coil,— Fig. 151 represents an instrument in whi
Davis's manual of magnetism : including galvanism, magnetism, electro-magnetism, electro-dynamics, magneto-electricity, and thermo-electricity . a steel mag-net is placed in a proper po-sition near the revolving bar,^ ^ in either of the instruments whose motion depends on the earth^s influence, itrotates with much greater speed than by the actionof terrestrial magnetism alone. Its motion may bereversed, notwithstanding the opposing influence ofthe earth, by disposing the permanent magnet in asuitable manner. 347. Electro-Magnet revolving within a Coil,— Fig. 151 represents an instrument in which astraight electro-magnet revolves within a circular same current traverses the coil and the wire sur-rounding the rotating bar, but its direction is changedonly in the latter. When the circuit is completed,the revolving bar moves so as to bring its poles intoa direction corresponding with those of the motion depends upon the same cause as thatof a galvanometer needle, (^ 158.) As the poles ofthe coil are situated in the line of its axis, thesprings pressing on the segments of the pole-changer19 218 DAVIS S are so arranged as to reverse the current when theelectro-magnet is at right angles with the coil. TheFia- 151 north pole of the magnet places itself within the north pole of the coil, andnot, as might at first sight be ex-pected, within its south pole. Thisapparent anomaly is due to the circum-stance of the magnet not being pre-sented to the coil from the outside,but lying within, with its centre coin-ciding with that of the coil. If amagnetic needle is brought up to ahelix, its north pole is attracted to-wards the south pole of the helix, (see 268), and passes within it, until its centre lies inthe centre of the coil. In this position, its poles liein the same direction as those of the electro-magnetdescribed above. 348. Double Revolving Magnet. — In this in-strument, represented in Fig. 152, there are two „. -_„ electro-maarnets of th
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Keywords: ., bookcentury1800, bookdecade1850, booksubjectmagnetism, bookyear18
