p 179
When the needle, by its sudden disturbance in its horary course, indicates
the presence of a magnetic storm, we are still unfortunately ignorant
whether the seat of the disturbing cause is to be sought in the Earth itself
or in the upper regions of the atmosphere. If we regard the Earth as a true
magnet, we are obliged, according to the views entertained by Friedrich
Gauss (the acute propounder of a generaltheory of terrestrial magnetism), to
ascribe to every portion of the globe measuring one eighth of a cubic meter
(or 3 7/10ths of a French cubic foot) in volume, an average amount of
magnetism equal to that contained in a magnetic rod of 1 lb. weight.*
[footnote[ *Gauss and Weber, 'Resultate des Magnet. Vereins', 1838, 31, s.
146.
If iron and nickel, and probably, also, cobalt (but not chrome, as has long
been believed),* are the only substances which become permanently magnetic,
and retain polarity from a certain coerceive force, the phenomena of Arago's
magnetism of rotation and of Faraday's induced currents show, on the other
hand, that all telluric substances may possibly be made transitorily
magnetic.
According to Faraday ('London and Edinburgh Philosophical Magazine', 1836,
vol. viii., p. 178), pure cobalt is totally devoid of magnetic power.
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