. The medical and surgical uses of electricity. . ctric in-duction. The first and most familiar form is when a current is induced in a coilof insulated wire. The second form is when a current is induced in con-ducting plates. Under electro-magnetic induction we have seen that the coil of wire inwhich a current circulates produces a contrary induced current in an ad-jacent coil whenever a change is made in the current by opening, closing,withdrawing, or approaching it. The strength of the induced current isproportioned to the amount and suddenness of these changes. If now wesubstitute for the p
. The medical and surgical uses of electricity. . ctric in-duction. The first and most familiar form is when a current is induced in a coilof insulated wire. The second form is when a current is induced in con-ducting plates. Under electro-magnetic induction we have seen that the coil of wire inwhich a current circulates produces a contrary induced current in an ad-jacent coil whenever a change is made in the current by opening, closing,withdrawing, or approaching it. The strength of the induced current isproportioned to the amount and suddenness of these changes. If now wesubstitute for the primary or inducing coil a permanent bar magnet, andcause it to approach or withdraw from the adjacent coil, it induces a cur-rent in that coil. This principle is the basis of all the magneto-electricmachines that are so familiar to students of philosophy, and that were onceso much used in electro-therapeutics. The development of magneto-electricity is shown in a very simple man-ner by the common horseshoe magnet, its armature, and a copper HISTORY OF INDUCTION. 57 Faradays disk machine (Fig. 36) consists of a copper disk rotated be-tween the poles of a permanent magnet. The current traverses from shaftto rim, or vice versa, according to rotation. In the electro-magnetic machines in ordinary use a soft iron armaturecovered with wire is made to rotate at the poles of a permanent horseshoemagnet. As the armature rotates, itstwo ends are, of course, alternatelybrought near to and removed from thepoles of the magnet, and thus two cur-rents are induced in the wires that l_cover the armature. Each current lastshalf of a revolution, and if the rotationbe rapidly kept up a current is pro-duced which may be perceived whenthe ends of the wires are joined. A Continuous Current from Mag- ^^^ ^^ fie to-Electric Machines.—When the armatures of the magneto-electric ma-chine are made to revolve with sufficient rapidity, a continuous current isproduced which has all the properties of th
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