321Bibliography
Mary Jo Nye · 2025
Pliny described ancient theories of the attractive power of some kinds of iron rocks for metals, but the first instrument for measuring magnetism was the compass device invented in ancient China. The compass needle appeared to be attracted toward celestial true North, providing a means of locating directions. By the fifteenth century, there was recognition that the needle deviates slightly from the Pole Star (declination), suggesting that the attracting power lies elsewhere, perhaps in magnetic rocks in the north or, as assumed by William Gilbert around 1600, in iron material within the Earth, since the needle dips from the horizon (inclination). The current dynamo theory that the Earth’s magnetic field originates in the Earth’s electrically charged molten core emerged in the mid-twentieth century from competing approaches taken by the physicists Patrick Blackett and Edward Bullard after Blackett disproved his own much updated version of Gilbert’s idea.