Terrestrial The statement usually made that a freely suspended magnet needle, remote from magnetic masses, tends to point north and south is not correct except for a few localities at certain times. The actual nature of the earth's magnetic field must be found by extended experiments which are being carried on by numerous observers largely under the direction of various governments. (See methods of magnetic measurements above). If a steel needle be suspended by a silk fibre and carefully balanced so as to hang horizontal and is then magnetized it will be observed to finally come to rest in a certain vertical plane and to be inclined to the horizontal. The angle be tween a horizontal line and the direction of the needle is called the dip, and the angle between the true north and south plane and that in which the needle lies is called the declination. The values of the dip, declination and intensity of the earth's field at a point are called the magnetic elements at that point. The use of the compass both by the surveyor and mariner over nearly the entire surface of the earth makes an accurate knowledge of these elements indispen sable. In order to convey this information, in a practical way, recourse is had to maps on which places having the same declination, for example, are joined by lines. Such maps bring into view many interesting features as regards the earth's magnetism. For example, Fig. 9, published by the United States Coast and Geo detic Survey for 1900 shows that in northern Oregon, Idaho and Montana the compass pointed approximately 20 degrees east, while in the extreme northeastern part of Maine it pointed about 20 degrees west. Along an irreg ular line crossing Michigan, Ohio, North and South Carolina and passing east of Cuba the declination was 0, or the needle pointed due north. It is evident from an inspection of these maps that the poles of the earth considered as a magnet do not coincide with the geographic poles. The line of no dip follows the equator only approximately. North of this line the i north end dips down, while at the south it •is reversed. Some of the minor variations are no doubt caused by local causes, such as masses of magnetic material, but it is a general belief among observers that the earth's magnetism is largely due to outside agencies. Another very important point for the mariner, who depends on the compass to find his way in safety across trackless seas or the surveyor anxious to locate landmarks, is that these magnetic elements are continually changing even during the day, as well as month by month and year by year. In London during 232 years the declination changed 35 degrees. CA street one mile long laid .out in London parallel to the compass direction in 1580 would have its terminus seven tenths of a mile too far east according to the compass in Since 1812 the declination at London has changed from about 24 degrees west to 16 degrees west. In 1580 it was 11 degrees east. In fact it would seem that the magnetic poles of the earth are slowly vibrating. The periods of some of the components of this vibration are astronomical in origin — the day, the year, the lunar month, the sun-spot period, etc. The variation during the day must be taken into account in accurate work as a mile run in the morning and repeated in the after noon may vary by 5 to 20 feet at its terminus.
Sudden changes called magnetic storms also frequently occur, which seem to be associated with atmospheric electrical conditions, sun spots, etc. In order to secure data for the study of these complex phenomena, magnetic observa tories are maintained where delicate instruments record, day and night, the countless fluctuations of the magnetic forces.
The problem of the navigator is still further complicated by the use of iron ships which are always sources of disturbance, both because of their permanent as well as their variable mag netism. The continual jarring and changes of temperature during a voyage enables the earth's field to continually change the distribution of magnetism in the vessel. The means to be used for the correction of this deviation have re ceived the attention of many skilful investi gators. The limits of this article will hardly allow a discussion of the matter which may be found in special books noted at the end.
The contrast between the state of knowledge regarding magnetism before 1600 and its pres ent development is one of the most striking in dications of the growth of scientific investiga tion. Instead of vague speculations, partial truths veiled in mysticism, more or less direct references to dogma and the supernatural, we have organized knowledge based on experience and constantly checked by experiment and ap plication. The number of those who believe in ((magnetic" healing or, that, because an iron pipe driven in the earth shows polarity, the water flowing through it is magnetic and has special medicinal virtues, is constantly on the decrease. The relations between magnetism and other fields of physical research cannot be treated in this article, yet it may be well to mention that such relations are constantly being investigated and no one in touch with present developments believes that the end is at hand. And it may well be that the delicately poised magnetic needle in some future interpretation of its count less movements will give us a knowledge of the invisible yet all-pervading agency which governs its fluctuations and lead us to a broader generalization of physical phenomena than we can formulate at present. (See ELECTRICITY, ELECTROMAGNETISM, etc.). For the optical ef fects of magnets in rotating the plane of polari zation of light see LIGHT, POLARIZATION.
Crapper, 'Electric and Magnetic Circuits' (1903) ; Du Bois, 'The Magnetic Circuit in Theory and Practice' (1894) ; Ewing, 'Magnetic Induction in Iron and Other Metals> (1892) ; Fleming, J. A., 'Magnets and Electric Currentb' (London 1902) ; Gilbert, 'The Loadstone and Magnetic Bodies,> reprint (1900) ; Jeans; J. H., 'Elec tricity and Magnetism' (Cambridge 1911) ; Kelvin, (Reprint of Papers on Electricity and Magnetism) (1892) ; Lodge, (Modern Views of Electricity' (1889) •, Lyons, 'A Treatise on Electro-Magnetic Phenomena' (1903); Von Helmholtz, 'Wissenschaftliche Abhandlungen' (1882) also numerous textbooks on physics, electricity, electrical machinery, etc. For ter restrial magnetism consult the periodical Ter restrial Magnetism and Atmospheric Electri city, published at Baltimore.