0000 FIG. 49. P of constraint: they become polarized, their opposite sides having opposite properties, and, as each particle induces a state of polar tension in its neighbor, the effect is transferred to a distance. In Fig. 49, P represents a positively charged body, and a b c d intermediate particles of air. These are thrown into opposite states or polarized, as is represented by the white and black sides of the spheres, and thus the effect is propagated to the body N, which is electrically excited. We have said that insulators arrest electricity, but on this view they only stop movement by conduction; they transmit it by induction through the polarization of their particles. As the polar particles are in active relations of force to those around, it is obvious that the effects may be propagated in various directions. Hence the polarization may occur in curved lines, and induction take place round corners and behind obstacles. N Polarization of Atoms. § 2. Magnetism. 128. Natural and Artificial Magnets. If a fragment of iron-ore called the loadstone is suspended, it turns one of its sides to the north, and the opposite to the south; it attracts to itself particles of iron or steel, and is called a natural magnet. If a steel bar be rubbed by a natural S FIG. 50. I Magnetic Needle. magnet, it acquires magnetic properties, and becomes an artificial N magnet. If properly shaped and poised upon a pivot, Fig. 50, it takes a northerly and southerly direction. The extremity which points northward is called the north pole of the magnet, and that which turns southward, the south pole. 129. Polarity.-If a second needle be brought near the first, it will be noticed that they exert a powerful influence over each other. The north pole of each attracts the south pole of the other, while north pole repels north pole, and south pole repels south pole. In short, like poles repel, and unlike attract each other. These influences are exerted through all kinds of matter-glass, wood, metals, or the human body—and through a vacuum. The magnetic force is manifested chiefly at the poles. If a sheet of FIG. 51. Magnetic Curves. netic curves, Fig. 51. Thus the two magnetic forces are always produced simultaneously; are equal in amount, but opposite in direction; and as these opposite powers are manifested in the poles of the magnet, they are called polar forces, while the property excited is termed polarity. FIG. 52. FIG. 53. 130. Magnetic Induction.-The preceding experiments show that the magnet has the power of exciting magnetism in adjoining bodies; in fact, each of the little particles of iron becomes a magnet with a north and south pole. This may be proved by placing several bars of soft iron around the pole of a magnetic bar, Fig. 52, when they all become temporarily magnetic. The permanent magnet N S S induces the influence in Magnetic Induction. S N N Magnetic Chain. the adjacent bars, which are hence said to be magnetized by induction. A key may be suspended by a magnet, Fig. 53, and this will hold a second smaller key, this a nail, and the nail a tack, all receiving their magnetism by induction from the bar, and each possessing its separate north and south polarity. FIG. 54. 131. Polarity of Particles.-Now, the particles of the magnet are in the same condition as the magnet itself. If a magnet is broken, as in Fig. 54, and the pieces are broken again and again, the smallest particles still have opposite poles. Each particle acquires polarity, and acts by induction upon all the others, the opposite pow S S NS NS A Broken Magnet. N N ers becoming accumulated at the opposite extremities of the bar. It may be observed that while steel retains its magnetism—that is, its particles remain fixed in their polar relation-soft iron, on the contrary, only remains a magnet while immediately acted upon. 132. Diamagnetism.-Magnetic bars are usually bent in the shape of a horseshoe, and their poles are connected by a piece of iron called an armature. The space between FIG. 55. Diamagnetism. the poles is called the magnetic field; a line joining the poles the axis; the line at right angles with this, the equator. All substances which, when freely suspended between the poles, of a magnet, arrange themselves axially, are classed as magnetic. Iron, nickel, cobalt, and oxygen, are the most important. Certain bodies, when suspended in the magnetic field, assume an equa torial direction, as if repelled by the poles, and these are said to be diamagnetic. In Fig. 55, 6 represents a bar of diamagnetic bismuth suspended by fibres of unspun silk between the two poles of a magnet. This property is also manifested by antimony, wood, leather, water, etc.; in fact, all substances not magnetic are now regarded as diamagnetic. 3. Voltaic Electricity. 133. The Voltaic Circuit.-We are now to consider electricity in a form more closely related to chemical action. It was first discovered by Galvani, and has been called after him galvanism; but its most illustrious cultivator was Volta, from whom it is also called voltaic electricity. A strip of zinc and one of copper are placed in a vessel containing water, to which has been added a little sulphuric acid. If not permitted to touch each other, as in Fig. 56, there is no effect. But, if brought into contact, as seen in Fig. 57, several results ensue. The acid in the water grows weaker; the zinc strip is corroded, and bub bles of gas are seen to escape from the surface of the copper. If the metals are separated, the action ceases; and, if this is done in the dark, a minute spark will be seen. Electricity seems to flow round and round in the direction of the arrows, like an invisible stream. The combination through which it passes is termed a voltaic circuit, and the circulating force an electric or electromotive current. The electric current originates in chemical changes, and requires a compound liquid capable of decomposition by one of the metals. The source of the electricity, in this case, is the decomposition of the sulphuric acid forming zinc sulphate, and setting free hydrogen gas. The zinc sulphate being dissolved in the liquid, the plate is kept clean and the action maintained, till the metal is consumed, or the acid all neutralized. 134. Electrodes.-To the plates are often soldered wires with terminals of platinum to withstand the action of corrosive liquids. The ends of these wires are known as the poles of the circuit, from an idea that they exerted an attractive and repellant action, like the poles of a magnet. But Faraday has proved that there is no attraction or repulsion in the case, and suggested the better term electrodes, which means simply a door or way for the electricity. The copper pole is termed positive, and the zinc pole negative. Whatever be the metals used, that which is chemically acted upon and originates the electricity is termed positive. FIG. 58. 135. The Voltaic File.-The power of the circuit may be increased by repeating its elements. The pile discovered by Volta and named after him was the first contrivance for augmenting the force of the electric current. It is made by preparing small plates or disks of metal, usually copper and zinc, and placing between them pieces of flannel moistened with an acid or saline solution. c Such a pile is represented in Fig. 58. The cloth is placed between the metals, and the order begun is preserved. Commencing at the bottom there is copper (c), flannel (ƒ), zinc (z), and upon that copper, flannel, zinc, and so on to fifty or a hundred sets, as may be desired (Fig. 58). The lower or copper end is positive, and the other negative; a current, therefore, moves in the direction of the arrows. This form of instrument gives a strong effect at first, but rapidly declines in power, Voltaic Pile. 136. The Galvanic Battery.-To augment the electrical |