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5.   What was known before Gilbert
about Magnets and the Compass Needle

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1. De Magnete
    Review #1

2. De Magnete
Review #2

3. More on "De Magnete"

4. A Gilbert Expt.

5. Before Gilbert

6. London in 1600

7. 1600-1820

8. Oersted & Ampére

9. The Lodestone

10. Gauss

    The ancient Greeks knew about lodestones (sometimes spelled loadstones), strange minerals with the power to attract iron. Some were found near the city of Magnesia in Asia Minor (now Turkey), and that city lent its name to all things magnetic.

    The early Chinese also knew about lodestones and about iron magnetized by them. Around the year 1000 they discovered that when a lodestone or an iron magnet was placed on a float in a bowl of water, is always pointed south. From this developed the magnetic compass, which quickly spread to the Arabs and from them to Europe. The compass helped ships navigate safely, even out of sight of land, even when clouds covered the stars. Compasses were also built into portable sundials, whose pointers had to face north to give the correct time.

    The nature of magnetism and the strange directional properties of the compass were a complete mystery. For instance, no garlic was allowed on board ships, in the mistaken belief that its pungent fumes caused the compass to malfunction. Columbus felt the compass needle was somehow attracted by the pole star, which maintained a fixed position in the northern sky while the rest of the heavens rotated around it.

      Drawing in Gilbert's book
      showing the downward slant
      of the magnetic force
    Norman used a compass needle balanced on a horizontal axis, able to swing in a vertical plane lined up in the north-south direction. That way, the north pointing end of the needle could still point northwards, and it did so, but it was no longer horizontal. Instead it slanted downwards at a steep "dip angle." This demonstrated that the magnetic force was not horizontal, either.

Questions from Users: ***           Geomancy
                  ***   Measuring the Earth's Magnetic Field
            ***   Why is southern end of compass needle heavier?

Next Stops:   1. A review of "De Magnete"

                    2. Another review of "De Magnete"

About the Background:   6. London in 1600

Further reading: The Riddle of the Compass by Amir D. Aczel is a popular book on the introduction of the compass to navigation (before 1600) and the history surrounding it; xvii + 178pp, Harcourt Inc. 2001.

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Author and Curator:   Dr. David P. Stern
     Mail to Dr.Stern:   earthmag("at" symbol)phy6.org

Last updated 1 March 2007
Re-formatted 18 March 2006