Acknowledgements Of The Strength Of A Magnet

This small book began during a Wednesday hiking group. Walter Page and my other nontechnical friends often asked questions about my lifelong specialty, the natural magnetic fields of the Earth. I had to find ways to explain these phenomena without equations and to answer types of questions that rarely arise from my more scientific colleagues. I so enjoyed this regular Wednesday challenge that this book developed quite easily.

Although most illustrations are of my own creation, I thank the many organizations that provided special figures for this book, in particular the National Geophysical Data Center of NOAA, the Space Environment Center of NOAA, the Geomagnetism Section of USGS, and the Goddard Space Flight Center of NASA.

Finally, I thank my wife, Beth, for proofreading the manuscript and tolerating my time at the computer, away from household duties.

E Chapter 1

Nature's Magnetism

We live on this Earth in a magnetic field environment that influences our daily lives in a variety of ways. In this guided tour we will explore some of the significant magnetic field effects and debunk some magnetic field myths. Along the way, acting as your guide, I will point out and illustrate how the sources of these natural magnetic fields change in time and place.

1.11 Finding the Fields

Although most of our knowledge of the magnetic field is acquired indirectly, this is not an unusual route to understanding. Natural phenomena such as wind or rain are sensed directly as the strong blast of air hits our face or the soaking rain covers our head. But we also identify these familiar processes indirectly as we look through a window and see a flag waving in the wind or hear rain hitting the rooftop of our house. We know that there is a gravity field because its magnitude and direction are sensed as it pulls us down the stairs or restrains the weight we wish to lift. In contrast, we have no obvious body sensors that can respond to magnetic fields to tell us its strength and direction. The closest direct sensation of magnetic fields for us is the push or pull we experience when we draw two magnets close together. Fortunately, indirect indications of magnetic fields abound in nature—we examine that evidence in our tour.

[1.21 Historical Tour Markers 1.2.1 Compass Origin

The word magnet is thought to be derived from Magnesia, a place of natural abundance for magnetic material in a region of ancient Macedonia. Plato wrote of the magnetic attraction of certain rocks that was well known to Greeks about 400 BC. Written records show that a Chinese compass, Si Nan, had already been fabricated between 300 and 200 BC and used for the alignment of constructions to be magically harmonious with the natural Earth forces. The Chinese fashioned their magnetized rock into a ladle-like shape, corresponding to our Big Dipper constellation (which the astronomers call Ursa Major). For ages it had been known that the last two stars on the bowl, opposite the handle of the Big Dipper, point toward the North Star. Similarly, the Chinese designed their spoon compass so that the bowl's outer lip would point in the horizontal northward direction (Figure 1.1). Then, the magnetic spoon balanced on its heavy rounded cup so that the lighter handle pointed toward a southward compass direction. Chu Yu, a Cantonese author of 1117, told of Chinese ship pilots using a compass for steering their ships in overcast cloudy conditions.

NORTH STAR

FIGURE 1.1 ► The Chinese spoon compass was carved from magnetite to resemble the Big Dipper constellation so that both bowls pointed in the northward direction.

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