1912 Read Free Page B

was needed, to test the scientific community’s understanding of the planet’s magnetism and exploit it. More accurate hydrographic surveys and maps showing magnetic field irregularities would improve the accuracy of navigation, reducing passage times and preventing disaster—and helping in sovereignty claims. The pressure for this data only increased with the shift from wooden craft to metal shipping, further distorting the magnetic signal.
    The first to raise the issue was Robert Norman, who in 1581 published a book called The New Attractive after he became frustrated at the way compass needles would incline below the ‘plaine of the horizon’: no matter how carefully he prepared his needles in London, once they were magnetised the north-facing part would dip without fail. Norman found he had to snip the end off the north-seeking part of his needles, thereby allowing them to balance on the pivot. He went on to measure this effect by setting up a magnetised needle vertically and reading the angle of magnetic dip. His dipping needle showed they always pointed to 72°. Norman felt it was something inherent to the needles themselves.
    In 1600 Queen Elizabeth I’s physician and scientist, William Gilbert, proposed a different, revolutionary idea. It was not the needles themselves that caused the dip, Gilbert argued. Instead, the phenomenon could best be explained if the planet had something akin to a powerful bar magnet inside it. Gilbert did not understand the cause, but we now know the magnetic field is produced by a solid inner iron core surrounded by fluid iron. Itis this outer part that acts like a spinning conductor in a bicycle dynamo. Rather than frantically peddling, though, the Earth’s system is run by heat from the decay of radioactive elements left over from our planet’s formation. The resulting swirling molten iron in the outer core is electrically charged, creating a continuously changing electromagnetic field.
    The upshot of all this is that a freely hanging magnetised needle will align itself to the line of magnetic force. Scatter iron filings on a sheet of paper covering a bar magnet and the filings will rapidly align themselves to the magnetic field, tracing a semicircle of iron around the bar, connecting the poles at either end. Depending where on the Earth’s surface you stand, the strength and direction of the horizontal and vertical parts of the magnetic field will vary. In the tropics the horizontal force dominates the magnetic field, so a needle will tend to sit parallel to the surface. But, as Robert Norman found, approaching polar regions the amount of dip increases as the field sweeps around the Earth and returns to the magnetic poles. As a result, a needle in their vicinity will approach a more vertical position. Norman’s dipping compass enabled people to measure the vertical part of the field, and it could also be an asset for navigation, providing a measure of latitude and, ultimately, proximity to a magnetic pole.
    The magnetic core is tilted at a slight angle off the axis of our planet’s rotation, by some 11°. However, although a bar magnet in the Earth is a great concept, it is only an approximation of what is going on under our feet. Swirling molten currents, the magnetism of surrounding rocks and changes in the sun’s activity all complicate this notion of a simple magnetic field. The result is one of the more perplexing concepts in Earth science: the presence of two different types of magnetic poles in each hemisphere. The better-known magnetic poles are where the field dips at right angles to the surface, while thegeomagnetic poles are the theoretical locations for the axis of the Earth’s magnetic field if it did truly work like a bar magnet, as William Gilbert envisaged. In each hemisphere these poles are more than a thousand kilometres apart, and over time change their absolute and relative positions to one another as they move