Davis's manual of magnetism : including galvanism, magnetism, electro-magnetism, electro-dynamics, magneto-electricity, and thermo-electricity . oil needle will now be deflected in one directionwhen an armature is applied to the poles, and in theopposite direction when it is removed. 433. The most powerful effects are obtained bycausing a bar of soft iron, enclosed in a helix, torevolve, by mechanical means, near the poles of asteel magnet. The principle may be illustrated byconnecting the cups, A and B, of the revolving elec-tro-magnet (Fig. 146) with a delicate galvanometer,as
Davis's manual of magnetism : including galvanism, magnetism, electro-magnetism, electro-dynamics, magneto-electricity, and thermo-electricity . oil needle will now be deflected in one directionwhen an armature is applied to the poles, and in theopposite direction when it is removed. 433. The most powerful effects are obtained bycausing a bar of soft iron, enclosed in a helix, torevolve, by mechanical means, near the poles of asteel magnet. The principle may be illustrated byconnecting the cups, A and B, of the revolving elec-tro-magnet (Fig. 146) with a delicate galvanometer,as represented in Fig. 172. On causing the iron barto rotate rapidly, by drawing the hand over the axis,the needle is powerfully deflected. As the iron ap-proaches the poles of the steel magnet, it becomesmagnetic; as it recedes from them, its magnetismdisappears. While one end of the bar is leaving thenorth pole, or approaching the south pole, the electriccurrent flows in a constant direction, the loss of south 268 DAVISS MANUAL. polarity by the iron, and the production of northpolarity in it, inducing currents in the same direc- Fig, tion. During the other half of the revolution, thesame end of the bar is approaching the north poleor receding from the south, and a reverse current isproduced. The two currents are conveyed to thegalvanometer in one direction by means of the pole-changer on the axis of the revolving bar. The pole-changer, in place of altering the direction of a currentconveyed to the coil, as it does when the bar ismade to revolve by a galvanic battery, here performsthe duty of bringing two opposite currents from thecoil into one course. The current traversing thewire of the galvanometer is, consequently, not re-versed, except by changing the direction of therotation. MAGNETO—ELECTRIC MACHINE. 269 434. If the cups A and B are connected with thecups c and c of the double helix and electrotome,slight secondary shocks, which may sometimes befelt in the hands,
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Keywords: ., bookcentury1800, bookdecade1850, booksubjectmagnetism, bookyear18
