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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| History of geomagnetism | 2/4 | https://en.wikipedia.org/wiki/History_of_geomagnetism | reference | science, encyclopedia | 2026-05-05T16:17:55.648373+00:00 | kb-cron |
=== Inclination === A compass must be balanced to counter the tendency of the needle to dip in the direction of the Earth's field. Otherwise, it will not spin freely. Often, compasses that are balanced for one latitude do not work as well at a different latitude. This problem was first reported by Georg Hartmann, a vicar in Nuremberg, in 1544. The English mariner Robert Norman was the first to recognize that this occurs because the Earth's field itself is tilted from the vertical. In his book The Newe Attractive, Norman called inclination "a newe discouered secret and subtil propertie concernyng the Declinyng of the Needle." He created a compass in which the needle was floated in a goblet of water, attached to a cork to make it neutrally buoyant. The needle could orient itself in any direction, so it dipped to align itself with the Earth's field. Norman also created a dip circle, a compass needle pivoted about a horizontal axis, to measure the effect.
== Early ideas about the source ==
In early attempts to understand the Earth's magnetic field, measuring it was only part of the challenge. Understanding the measurements was also difficult because the mathematical and physical concepts had not yet been developed – in particular, the concept of a vector field that associates a vector with each point in space. The Earth's field is generally represented by field lines that run from pole to pole; the field at any point is parallel to a field line but does not have to point at either pole. As late as the eighteenth century, however, a natural philosopher would believe that a magnet had to be pointing directly at something. Thus, the Earth's magnetic field had to be explained by localized sources, and as more was learned about the Earth's field, these sources became increasingly complex. At first, in both China and Europe, the source was assumed to be in the heavens – either the celestial poles or the Pole star. These theories required that magnets point at (or very close to) true north, so they ran into difficulty when the existence of declination was accepted. Then natural philosophers began to propose earthly sources such as a rock or mountain. Legends about magnetic mountains go back to the classical era. Ptolemy recounted a legend about magnetic islands (now thought to be near Borneo) that exerted such a strong attraction on ships with nails that the ships were held in place and could not move. Even more dramatic was the Arab legend (recounted in One Thousand and One Nights) that a magnetic mountain could pull all the nails out of a ship, causing the ship to fall apart and founder. The story passed to Europe and became part of several epic tales. Europeans started to place magnetic mountains on their maps in the sixteenth century. A notable example is Gerardus Mercator, whose famous maps included a magnetic mountain or two near the North Pole. At first, he just placed a mountain in an arbitrary location; but later he attempted to measure its location based on declinations from different locations in Europe. When subsequent measurements resulted in two contradictory estimates for the mountain, he simply placed two mountains on the map.
== Beginnings of modern science ==
=== William Gilbert === Magnus magnes ipse est globus terrestris. (The Earth itself is a great magnet.) 1600 was a notable year for William Gilbert. He became president of the Royal College of Physicians of London, was appointed personal physician for Queen Elizabeth I, and wrote De Magnete, one of the books that mark the beginning of modern science. De Magnete is most famous for introducing (or at least popularizing) an experimental approach to science and deducing that the Earth is a great magnet. Gilbert's book is divided into six chapters. The first is an introduction in which he discusses the importance of experiment and various facts about the Earth, including the insignificance of surface topography compared to the radius of the Earth. He also announces his deduction that the Earth is a great magnet. In book 2, Gilbert deals with "coition", or the laws of attraction. Gilbert distinguishes between magnetism and static electricity (the latter being induced by rubbing amber) and reports many experiments with both (some dating back to Peregrinus). One involves breaking a magnet in two and showing that both parts have a north and south pole. He also dismisses the idea of perpetual motion. The third book has a general description of magnetic directions along with details on how to magnetize a needle. He also introduces his terella, or "little Earth". This is a magnetized sphere that he uses to model the magnetic properties of the Earth. In chapters 4 and 5 he goes into more detail about the two components of the direction, declination and inclination.