274 MAGNETISM exists it is less than 0-000033 second. No explanation has been given of the discrepancy of these results. Direct In the early part of this century there was an animated influence controversy as to whether light exerted a direct influence of light U p 0n magnetization, in which Morichini, Mrs Somer- magnet- v ^ e > Christie, Eiess and Moser, and many others took ization. part. Nothing definite, however, was established. A similar fate befell the attempts to trace the influence of magnetic force upon crystallization, and to detect a relation between magnetism and gravity, 1 although both quests at one time or another engaged the skill of Faraday. FORMS, CONSTRUCTION, AND PRESERVATION OF MAGNETS. This subject occupied a large portion of most of the earlier treatises on magnetism. Much of the information given, however, either has now been recognized to be of questionable value or has been superseded by recent pro gress, and retains a merely antiquarian interest ; a few brief remarks, mainly historical, will therefore be sufficient. The oldest form of magnet was a piece of magnetic iron ore or loadstone. The power of these natural magnets varied exceedingly from one specimen to another. An elaborate discussion of the various kinds of loadstone will be found in Gilbert s De Afagnete. 2 In order to increase the carrying power, the loadstone was usually N fitted with armatures of soft iron upon its polar regions ; figure 45, taken from Gilbert, represents one of the oldest arrangements. Fig. 45. Figure 46 is taken from a loadstone in the collection of physical apparatus belonging to the university of Edinburgh, the carrying power of which is 205 lb. A loadstone in the Teylerian Museum at Haarlem has a carrying power of 230 R> ; and one at Lisbon, pre- Load- stones. Fig. 46. sented by the emperor of China to King John V. of Portugal, is said to support as much as 300 R>. Small loadstones are often very powerful in proportion to their weight ; e.g., Newton is said to have worn in a ring one that weighed only 3 grains, and yet was able to carry about 746 grains ; and one in the physical collection at Edin burgh, formerly belonging to Sir John Leslie, weighing itself 3^ grains, had at one time a carrying power of 1560 grains. Steel The introduction of steel magnets, and the perfection to which magnets, they were gradually brought, caused the loadstone to fall into disuse. It is said that Galileo possessed the art of making steel magnets about the beginning of the 17th century. It was early discovered that the earth s force could be utilized in magnetizing steel. 1 For the literature, see Wiecl, Galv., 688, 689. 2 See also Gehler s Physikalisches Worterluch, art. "Magnetismus. " Gilbert was aware that a feeble magnetization could be produced in this way; and Michell, in his treatise on artificial magnets, minutely describes how weak magnets may be made by means of the earth s force, then combined into bundles or "magazines" and used in turn to produce stronger magnets, these used to produce still stronger, and so on. The earliest process of all was no doubt the method of rub bing or touching by another magnet. This method of making magnets was studied with much attention by the natural philosophers of the 18th century, among whom we may men tion Savery, Knight, Duhamel, Le Maire, Canton, Michell, jEpinus, Coulomb, and Euler. The method of single touch Single consists simply in stroking the bar to be magnetized alternately touch. on its two halves with the south and north poles of a loadstone or bar magnet, the stroke beginning always at the middle and ending at the end. According to Lament, the best plan is to lay the magnet flat, overlapping one half of the bar to be magnetized, and then draw it off ; when the magnet is held perpendicular to the bar during the process, the result is apt to give an irregular magnetiza tion : e.g. , we may even get a magnet with its two er.ds north poles and with a south pole in the middle, or one with four poles, a north and south pole at the two ends and a south and north pole in the middle. 3 The first improvement on single touch was double touch with Double separate magnets. This consists in using two magnets simul- touch. taneously on the two halves of the bar undergoing magnetization. The north pole of one and the south pole of the other are placed either close together, or at a small distance apart near the middle of the bar, and then each is drawn towards the end of the half on which it lies ; according to Lament, here, as in single touch, the magnets should be laid flat on the bar. 4 Michell introduced the further improvement of using two bar magnets (or bundles of such) fastened together and kept parallel at a small distance apart by means of small pieces of wood, the north pole of one being conter minous with the south pole of the other. This pair is placed verti cal with one end on the middle of the bar, drawn towards one end and slipped off, then replaced on the middle and drawn to the other end, and so on alternately until the moment of the bar ceases to increase any farther. Instead of the pair of bar magnets a horse shoe magnet might of course be used. Le Maire 8 introduced the essential improvement of placing the bar to be magnetized upon a larger bar, and then magnetizing the two together. The advantage of this is best seen in the form of the same device adopted by Canton 6 and Duhamel, 5 who mag netized steel bars in pairs, connecting them up parallel to each other by means of two pieces of soft iron, and then magnetizing them in opposite directions. It is easy to see that the magnetiza tion of the one reacts on the magnetization of the other and strengthens it. Michell 7 obtained a similar advantage by magnetizing a number of bars placed end to end in a line ; he found, as was to be expected, that the end bars were weaker, but this defect he remedied by repeating the process with the bars arranged in a different order. Coulomb s method was to place the ends of the bar on the north and south poles of two bar magnets arranged in line at the proper distance apart. This process of connecting up the bars to be magnetized in a closed magnetic circuit is sometimes called circular touch ; it can be applied to horse-shoe magnets by placing a pair of them with their ends together, and then passing round and round upon them a horse-shoe magnet or a pair of bar magnets arranged as already described. 8 Immediately after (Ersted s discovery of the magnetic action of Electro the galvanic current, Arago, 9 Boisgiraud, 10 and Davy almost simul- magnel taneously applied this property to the magnetization of iron and steel. 11 Powerful electromagnets, with cores of soft iron, were first constructed a few years later by Sturgeon and Brewster. Pohl, Moll, and Pfaff in Germany, and Henry and Ten Eyck in America, may be mentioned as the most successful of the early constructors. One of the electromagnets of Henry and Ten Eyck reached a carrying 3 Poles situated abnormally in this way are called "consecutive points." 4 This method appears to have been invented by Knight (about 1740), and used in producing the powerful magnets for which he was famous. The secret of his process was never divulged by himself, but was published by Wilson after his death. See art, MA.GNETISM, 8th edition of Encyclopedia Britannica. 5 Mem. d. TAcad. d. Paris, 1745 and 1750. 6 Phil. Trans., 1751. 7 Treatise of Artificial Magnets, 1750. 8 For fuller information on the present subject, see Gehler s Physi kalisches Wiirterbuch, art. "Magnetismus," xv. 9 Ann. d. Chim. et d. Phys., 1820. 10 Phil. Trans., 1820-21. 11 The anomalous magnetization of needles by the discharge from Leyden jars had been observed earlier, but not properly understood.
See art, ELECTRICITY, vol. viii. p. 82.