Gillian Turner (1)

I sometimes forget the advance of Science is not all that smooth. Gillian Turner has written a nice popularization of the discovery and explanation of the Earth’s magnetic field, starting all the way back with the legendary Greek shepherd Magnes, who found his iron-nailed boots stuck to a lodestone. The use of a compass to find direction apparently dates back to 1st century AD China, but doesn’t arrive in Europe until much later; early explanations usually involved a large mountain of lodestone at the North Pole. Secular variation, diurnal variation, and the fact that the compass didn’t point exactly north were all discovered by the 1600s, and things settled down to map-making (according to Turned, Edmund Halley’s maps of magnetic variation were the first use of contour lines).


The next major event was the discovery of “polar wandering” and magnetic reversals in the 1950s. My childhood recollections are that there was no debate about the reality of magnetic reversals at the time; Turner, however, points out that the idea was almost universally dismissed by geophysicists, especially after Nobelist Louis Néel described a theoretical way to induce anti-sense paleomagnetism in volcanic rock (i.e., a way that a certain combination of mineralogy and cooling history could cause the north-seeking poles of magnetic domains in the rock to align on south rather than north). One of the early papers on magnetic reversals and sea-floor spreading was dismissed by a reviewer as “possibly suitable for cocktail party conversation but not for a serious scientific journal”.


However, eventually things worked out; more and more and more evidence came in from the field, and supercomputer power allowed construction of a dynamo model of the earth that would actually reverse now and then (although it took 2000 hours on a Cray C90 for the model to do it). Both magnetic reversals and “polar wandering” were crucial in the development of plate tectonics theory. The model only rarely results in a reversal; most of the time the field drops to zero (well, the dipole field drops to zero but other components do not) and then returns in the same direction. Turner speculates briefly on what might happen if the dipole field disappears; do animals that depend on magnetic fields for navigation get lost? Do we get aurorae all the way to the equator? – but doesn’t go anywhere with this.


Interesting things learned on the way:


Magnetite (“lodestone”) only becomes magnetic (i.e., a magnet itself rather than just being attracted to a magnet) if struck by lightning.


The difference between ferromagnetism (all the magnetic domains in the material are aligned the same way), antiferromagnetism ( adjacent magnetic domains are aligned in opposite directions), ferrimagnetism (domains are aligned in both directions, but there is a preponderance in one direction so the material is weekly magnetic), dimagnetism (when an external magnetic field is applied, the material develops an induced magnetic field in the opposite direction) and paramagnetism (when an external magnetic field is applied the material develops an induced magnetic field in the same direction).


Light and quick reading; no math. Many illustrations. The bibliography seems a little weak and is mostly other “popular” works. The author is a lecturer at the University of Wellington in New Zealand and, according to the jacket blurb, is “...winner of numerous awards for excellence in teaching and science communication”; based on this book they’re well deserved.… (mehr)