On atmospheric magnetism / Professor Faraday.

Michael Faraday

Date:: [1851]

Credit: On atmospheric magnetism / Professor Faraday. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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|1851.J curves, or lines of magnetic force, existing in the space around. These phrases have a high meaning, and represent the ideality of magnetism. They imply not merely the directions of force, which are made manifest when a little magnet, or a crystal, or other subject of magnetic action is placed amongst them, but those lines of power which connect and sustain the polarities, and exist as much when there is no magnetic needle or crystal there as when there is; having an [independent existence analogous to (though very different in nature from) a ray of light or heat, which, though it he present in a given space, and even occupies time in its transmission, is absolutely insen- sible to us by any means whilst it remains a ray, and is only made known through its effects when it ceases to exist. The form of a line of magnetic force may vary exceedingly from a straight line to every degree of curvature, and may even have double and compli- pated curvatures impressed upon it. Its direction is determined by its polarity, the two changing together. Its powers are such, that a magnetic needle placed in it finds its place of rest parallel to it; a crystal of calcareous spar turns until its optic axis is transverse to it; and a wire which is unaffected when moved in or along it, has an electric current evolved the instant that it passes across it: by these and by other means the presence of the magnetic line of force and its direc- tion are rendered manifest. I The Earth is a great magnet : its power, according to Gauss, being equal to that which would be conferred if every cubic yard of t contained six one-pound magnets; the sum of the force therefore is Bqual to 8,464,000,000,000,000,000,000 such magnets. The dispo- iition of this magnetic force is not regular, nor are there any points m the surface which can be properly called poles : still the regions jif polarity are in high north and south latitudes ; and these are con- nected by lines of magnetic force (being the lines of direction) which, generally speaking, rise out of the earth in one (magnetic) hemi- sphere, and passing in varied directions over the equatorial regions .nto the other hemisphere, there enter into the earth to complete the mown circuit of power. A free needle shows the presence and direction of these lines. In London they issue from the earth at an [ingle of about 69° with the horizon (being the dip or inclination) ; ind the plane in which they rise forms an angle of 23“ W. nearly [with true north, giving what is called west declination. Where the pip is small, as at the magnetic equator, these lines scarcely rise out bf the earth and pass but a little way above the surface ; but where it is large, as in northern or southern latitudes, they rise up at a greater angle, and pass into the distant realms of space, from [whence they return again to the earth in the opposite magnetic hemi- sphere : thus investing the globe with a system of forces like that about an ordinary magnet, which wherever it passes through the atmo- sphere is subject to the changing action of its magnetic oxygen. fThere is every reason to believe that these lines are held in the earth, but of which they arise and by which they are produced, just as the

On atmospheric magnetism / Professor Faraday.

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