north, the "North-seeking" pole, and the other the "South-seeking" pole. We may therefore sum up our observations in the concise statement: Like magnetic poles repel one another; unlike poles attract one another. This we may call the first law of magnetism. 82. The two Poles inseparable.-It is impossible to obtain a magnet with only one pole. If we magnetise a piece of steel wire, or watch spring, by rubbing it with one pole of a magnet, we shall find that still it has two poles—one N.-seeking, the other S.-seeking. And if we break it into two parts, each part will still have two poles of opposite kinds. 83. Magnetic Force.—The force with which a magnet attracts or repels another magnet, or any piece of iron or steel, we shall call magnetic force.1 The force exerted by a magnet upon a bit of iron or on another magnet is not the same at all distances, the force being greater when the magnet is nearer, and less when the magnet is farther off. In fact the attraction due to a magnet-pole falls off inversely as the square of the distance from the pole. (See Art. 117.) B Whenever a force acts thus between two bodies, it acts on both of them, tending to move both. A magnet will attract a piece of iron, and a piece of iron will attract a magnet. This was shown by Sir Isaac Newton, who fixed a magnet upon a piece of cork and floated it in a basin of water (Fig. 44), and found that it moved across the basin when a piece of iron was held near. A compass needle thus floated turns round and points north and south; but it does not rush towards the north as a whole, nor towards the south. The reason of this will be explained later, in Art. 117. Fig. 44. 1 See footnote on "Force," Art. 155. Gilbert suggested that the force of a magnet might be measured by making it attract a piece of iron hung to one arm of a balance, weights being placed in the scalepan hanging to the other arm; and he found, by hanging the magnet to the balance and placing the iron beneath it, that the effect produced was the same. The action and reaction are then equal for magnetic forces. 84. Attraction across bodies. —If a sheet of glass, or wood, or paper, be interposed between a magnet and the piece of iron or steel it is attracting, it will still attract it as if nothing were interposed. A magnet sealed up in a glass tube still acts as a magnet. Lucretius found a magnet put into a brass vase attracted iron filings through the brass. Gilbert surrounded a magnet by a ring of flames, and found it still to be subject to magnetic attraction from without. Across water, vacuum, and all known substances, the magnetic forces will act; with the single exception, however, that magnetic force will not act across a screen of iron or other magnetic material. If a small magnet is suspended inside a hollow ball made of iron, no outside magnet will affect it. A hollow shell of iron will therefore act as a magnetic cage, and screen the space inside it from magnetic influences. 85. Magnetic Substances. - A distinction was drawn by Gilbert between magnets and magnetic substances. A magnet attracts only at its poles, and they possess opposite properties. But a lump of iron will attract either pole of the magnet, no matter what part of the lump be presented to the magnet. It has no distinguishable fixed "poles," and no magnetic "equator." A true magnet has poles, one of which is repelled by the pole of another magnet. 86. Other Magnetic Metals.-Later experimenters have extended the list of substances which are attracted by a magnet. In addition to iron (and steel) the following metals are recognised as magnetic :— Nickel. Manganese, Chromium. Cerium. and a few others. But only nickel and cobalt are at all comparable with iron and steel in magnetic power, and even they are very far inferior. Other bodies, sundry salts of iron and other metals, paper, porcelain, and oxygen gas, are also very feebly attracted by a powerful magnet. 87. Diamagnetism.—A number of bodies, notably bismuth, antimony, phosphorus, and copper, are repelled from the poles of a magnet. Such bodies are called diamagnetic bodies; a fuller account of them will be found in Lesson XXVIII. 88. The Earth a Magnet. - The greatest of Gilbert's discoveries was that of the inherent magnetism of the earth. The earth is itself a great magnet, whose "poles" coincide nearly, but not quite, with the geographical north and south poles, and therefore it causes a freely-suspended magnet to turn into a north and south position. The subject of Terrestrial Magnetism is treated of in Lesson XII. It is evident from the first law of magnetism that the magnetic condition of the northern regions of the earth must be the opposite to that of the north-seeking pole of a magnetised needle. Hence arises the difficulty alluded to on page 75. 89. Magnetic Induction. Magnetism may be communicated to a piece of iron, without actual contact with a magnet. If a short, thin unmagnetised bar of iron, be placed near some iron filings, and a magnet be brought near to the bar, the presence of the magnet will induce magnetism in the iron bar, and it will now attract the iron filings (Fig. 45). This inductive action is very similar to that observed in Lesson III. to take place when an electrified body was brought near a nonelectrified one. The analogy, indeed, goes farther than this, for it is found that the iron bar thus magnetised by induction will have two poles; the pole nearest to the pole of the inducing magnet being of the opposite kind, while the pole at the farther end of the bar is of the same kind as the inducing pole. Magnetism can, however, only be induced in those bodies which we have enumerated as magnetic bodies; and those bodies in which a magnetising force produces a high degree of magnetisation are said to possess a high co-efficient of magnetisation. It will be shown presently that magnetic induction takes place along certain directions called lines of magnetic induction, or lines of magnetic force, which may pass either through iron and other magnetic media, or through air, vacuum, glass, or other non-magnetic media: and, since induction goes on most freely in bodies of high magnetic susceptibility, those lines of force are sometimes (though not too accurately) said to "pass by preference through magnetic matter," or, that "magnetic matter conducts the lines of force." Although magnetic induction takes place at a distance across an intervening layer of air, glass, or vacuum, there is no doubt that the intervening medium is directly concerned in the transmission of the magnetic force, though probably the true medium is the "æther" of space surrounding the molecules of matter, not the molecules themselves. WATERSSC-V al magkama mages permanently. 12. ELI Jemely the Cast miss if we also AL INTEs imperfectly. Fare sat a showere, only The Lorme zemer Instrates I miner - 1 few pieces rari i se iron ane of these ef paced in conte pe of a permaNext Sta Te i is attracted Le ref a tem ATAY INTE Another bit of Je hum 3 L mà mth, a chain sheit m Surf the steel The SMYTH 20 The all the 5 LNG AS Tuninger magnetic. I se relies may be formed, but ium hore & see needle is more 2 BATEN ZI HIT reade of the same sir de matism into That Joh. And stir to get the magnetism * IS SLI NI I II: IT the steel retains the This power of resisting THE TRUSKI I ICSI is sometimes called |