North Pole, South Pole: The Quest to Understand Earth's Magnetism
304
North Pole, South Pole: The Quest to Understand Earth's Magnetism
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Product Details
| ISBN-13: | 9781927249109 |
|---|---|
| Publisher: | Awa Press |
| Publication date: | 05/01/2010 |
| Series: | Awa Science |
| Sold by: | Barnes & Noble |
| Format: | eBook |
| Pages: | 304 |
| File size: | 12 MB |
| Note: | This product may take a few minutes to download. |
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North Pole South Pole
The Epic Quest to Solve the Great Mystery of Earth's Magnetism
By Gillian Turner
Awa Press
Copyright © 2010 Gillian TurnerAll rights reserved.
ISBN: 978-1-927249-10-9
CHAPTER 1
The Mystery of Magnetism
Just as there are in the heavens two points more noteworthy than all the others ... so also in this stone ... there are two points, one north and the other south.
Petrus Peregrinus, 1269
Old Magnes had come this way so many times before that his feet knew every one of the black rocks over which he was clambering. Day after day, year after year, for most of his life he had trudged up this hillside to tend his small flock of sheep. Never before, though, had his iron-studded boots stuck to the rocks the way they were doing now. The only way he could get his foot free was with a mighty kick – and then, with his next step, his boot would be sucked down again.
His staff also seemed to have taken on a life of its own. Each time he planted it on a rock to steady himself he had to tug hard to lift it again. What was going on? Last night there had been terrifying thunder and lightning, followed by torrential rain – thank the gods he had come down to his shelter and not spent the night on the mountain – but the ground was now dry again and everything looked normal.
This legend of the Greek shepherd Magnes is thought to date back to around 900 BC, but it was recorded almost a millennium later by the Roman scholar and writer Pliny the Elder. Pliny was fascinated by the world around him, and before being killed by poisonous gases from Mount Vesuvius during the AD 79 eruption that destroyed Pompeii, he spent much of his life recording his observations of nature in a multi-volume encyclopaedia, Naturalis Historia. Pliny's story of Magnes, although no doubt embellished through centuries of retelling, provides two important clues to understanding Earth's magnetic properties: an electrical storm took place and rocks became magnetised.
Magnes was apparently climbing on Mount Ida – the same Mount Ida from which Zeus is said to have watched the sacking of Troy – in the north-west of what is now Turkey. This is not far from the region the ancient Greeks called Magnesia after their homeland in mainland Greece. Still today, Magnesia is well known for its deposits of lodestone, a rock that is rich in magnetite, an oxide of iron. Normally a lump of magnetite-bearing rock is unremarkable. However, if the rock is struck by lightning it becomes strongly magnetised. A bolt of lightning may pass an electric current of up to a million amps into the ground – not for long, but long enough for rocks within a short distance to become magnetised intensely and stably.
For centuries magnetism was thought to be unique to lodestone. What was it about lodestone, and only lodestone, the ancients wondered, that gave it this magical property? The earliest ideas on the nature and origin of magnetism are usually attributed to a Greek philosopher, Thales (c. 624 – 546 BC), who lived in Miletus, a busy trading city not far from Mount Ida.
Together with his well-known contemporary Pythagoras, Thales is credited with having laid the foundations of not just philosophy but also physics and mathematics. None of his original writings seem to have survived, but Aristotle reported:
Thales ... held soul to be a motive force ... he said that the magnet has a soul because it moves the iron.
The Greeks recognised that lodestones did not attract only other lodestones: they also attracted pieces of metallic iron. And they had observed that a piece of iron in contact with a magnet became magnetised itself, and so was able to attract another piece of iron – a process now known as induction.
Further, a lodestone did not need to be in physical contact with another lodestone or a piece of iron in order to attract it. This 'action-at-a-distance' effect, where a force acted across empty space in which no intermediary medium existed, seemed impossible to explain in material terms, so Thales reasoned that an animistic explanation was called for. Living bodies moved, and instilled motion in other material objects. Living bodies had souls. Therefore, in order to move a piece of iron the magnet, too, must possess a soul.
Thales was also familiar with another action-at-a-distance effect, namely that when a piece of amber was rubbed with fur it could attract scraps of chaff and other light particles. (This is the same 'electrostatic' effect that makes our hair crackle and stand on end after brushing it on a dry day.) However, whereas rubbed amber attracted scraps of all kinds of materials, lodestone attracted only other lodestones or iron.
These action-at-a-distance effects – which, as well as magnetic and electrostatic forces, also include gravity – would challenge not just Thales. Down the ages, scientists, philosophers, teachers and students would struggle to understand them, and create many and varied explanations.
Later Greek philosophers opted for an 'atomistic' view of matter. This bore little resemblance to modern atomic theory, other than the idea that matter was made up of innumerable tiny particles. In the fifth century BC, Diogenes of Apollonia maintained that a lodestone or magnet 'fed' on atoms of iron. Another school of thought believed that a magnet emitted particles, and that these particles cleared the space between it and a piece of iron, thus drawing them together.
This last idea led, over ensuing centuries, to a whole host of 'effluvia' theories involving invisible emissions from magnetic materials, and finally, in the nineteenth century, to the notion of the magnetic field. At this early stage, though, few theories addressed, let alone answered, the obvious question – why was magnetism confined to lodestone and iron?
Early Greek science was essentially limited to the observation of natural phenomena and endless philosophising as to their causes. Without the modern elements of prediction, experimentation and testing, alternative theories such as animist, atomist and effluvia could not be evaluated against each other in any substantial way, and so little progress was made.
* * *
At the same time as the Greeks were holding sway in the Mediterranean, an advanced civilisation was thriving in China. While science there was also inextricably mixed up with mysticism, divination and religion, technology reached a degree of sophistication that would be unparalleled in the West until the Renaissance of the fifteenth and sixteenth centuries.
The earliest recorded compass, a Chinese divining instrument, probably dated from the first century AD, although it could have been in existence as much as 300 years earlier. From early Chinese writings studied and described by the English historian of science Joseph Needham, we know that this compass was used to determine the directions favoured by the winds and waters, and so was a guide to laying out villages, building houses, ploughing fields, orienting tombs and much more – the ancient art of feng shui.
The instrument consisted of a spoon-shaped piece of lodestone, known in China as tzhu-shih or 'loving stone', which represented the star constellation of Ursa Major, the Great Bear. This was delicately balanced on a circular 'heaven' plate made of bronze or wood, which was itself placed on top of a square 'Earth' plate. Both the heaven plate and the Earth plate were intricately engraved with astronomical symbols and directions. The 'spoon' took on a natural magnetisation along its length so that, when balanced, its handle came to rest pointing to the south. Interestingly, the early Chinese routinely chose south as the prime cardinal direction.
The Chinese seem never to have questioned the nature of the force that aligned their compass. To them, as to the Greeks, such things lay in the lap of the gods. There is, however, documentary evidence that they recognised discrepancies between the compass's south and true south. Between about AD 720 and 1086, Chinese compasses appear to have deviated by up to 15° east of true south, while all later records show the deviation to have been to the west of true south. Indirect evidence of this deviation is to be found in the streets of many ancient Chinese towns and cities, including Beijing and Nanking. A plan of the southern part of the township of Shandan in Gansu province on the Old Silk Road shows two distinct street orientations. The older is due north – south, but the younger deviates by eleven degrees, trending from 11° west of south to 11° east of north. Presumably the streets were aligned to the favourable directions of the winds and waters as determined by the spoon-shaped compass, and between the two periods of building the compass had shifted to the west by eleven degrees.
These early Chinese were not great seafarers or travellers – had they been, the compass would almost certainly have become a navigational tool much earlier. As it is, the earliest reference to a mariner's compass comes from the beginning of the twelfth century. By then the Chinese had perfected techniques for magnetising a fine iron needle by stroking it with a piece of lodestone and balancing it on a finely made pivot, floating it on water, or suspending it from a fine silk thread in order to minimise the effect of friction and improve its overall performance. Beautiful floating fish and turtle-shaped pivoted compasses originate from this period.
Following the so-called Dark Ages, the compass eventually surfaced in Europe in the writings of an Englishman, Alexander Neckam. Born in St Albans in Hertfordshire in 1157 on the same night as Richard I, Neckam had grown up with the future king as a foster brother. He went on to teach arts at the newly emerging University of Paris and later returned to St Albans School before becoming a canon and abbot of the Augustinian abbey at Cirencester. Neckam's interests were far-ranging, from theology to natural philosophy, but he is remembered mainly for his two books, De Nominibus Utensilium (On Instruments), published around 1180, and De Naturis Rerum (On the Natures of Things), around 1200.
Each contained an article on nautical navigation. In the first, Neckam explains the use of a magnetic compass needle for navigation at sea, while in the second he extols the advantages of a pivoted needle:
The sailors moreover, as they sail over the sea, when in cloudy weather they can no longer profit by the light of the sun, or when the world is wrapped up in the darkness of the shades of night, and they are ignorant as to what point of the compass their ship's course is directed, they touch the magnet with a needle. This then whirls round in a circle until, when its motion ceases, its point looks direct to the north.
How the compass had reached Europe is something of a mystery. Neckam probably came across it first in Paris, but the tenor of his writing suggests that by the end of the twelfth century it was already in common use by mariners. This is at odds with the suggestion that it was Marco Polo who brought it back to Europe from China. Polo did not visit China until 1275 and returned to Venice in 1295, a whole century after Neckam's descriptions.
Another more persuasive theory is that the compass arrived in Europe courtesy of Arab traders. The presence of ancient Chinese objects through the Persian Gulf and Red Sea regions and along the east coast of Africa indicates that, from the eighth century onwards, there was busy trading between the Arab inhabitants and the Chinese. It is hard to imagine that the compass would not have eventually become an item of trade, and found its way north and west to Europe. The timing – one hundred years from the compass's invention in China to its appearance in Europe – is plausible. However, to complicate the theory, the earliest Arabic references to the compass also seem to post-date Neckam's. It is not until the mid thirteenth century that Arab documents and stone tablets mention sailors finding their way by means of floating compasses fashioned from fish-shaped pieces of iron rubbed with a magnet.
Perhaps the likeliest explanation is that the European compass was developed independently. This is supported by the difference in prime direction: to this day Chinese compasses are made with the prime end of the needle pointing south, while European compasses have always pointed to the north.
Neckam's reports and dates are backed up by a satirical poem, 'Bible', written by Guyot de Provins, a French poet and monk, around 1205. It includes the passage:
... there is an art which the sailors have, which cannot deceive. They take an ugly brown stone, the magnet, to which iron willingly attaches itself, and touching a needle with it, they fix the needle in a straw, and float it on the surface of water, whereupon it turns infallibly to the Pole Star.
Since Thales' time it had been known that a magnet attracted a piece of iron, while two magnets would attract or repel one another. A compass needle was just a magnet so what attracted or repelled it, and what caused it to rotate into a north – south alignment?
An interesting explanation emerged. Until the invention of the compass, the heavens had provided the chief means of navigation – the sun by day and the stars by night. It was commonly believed that the Earth lay at the centre of Creation, with the moon, sun, stars and known planets – Mercury, Venus, Mars, Jupiter and Saturn – arranged on crystal spheres of increasing size, each of which revolved around the Earth daily. Beyond Creation lived God in his heaven, and beyond this lay infinite space. One star, however, seemed to remain fixed in place, because it lay on the axis about which the celestial sphere of the stars revolved. And, as de Provins pointed out, it was towards this star that the compass needle infallibly turned. Hence, the directivity of the compass came to be attributed to the Pole Star.
An elaboration of the Pole Star theory appears in a poem by a thirteenth-century Italian, Guido Guinicelli:
In what strange regions 'neath the polar star
May the great hills of massy lodestone rise,
Virtue imparting to the ambient air
To draw the stubborn iron; while afar
From that same stone, the hidden virtue flies
To turn quivering needle to the Bear
In splendour blazing in the northern skies.
This brief verse captured several important ideas. Although all eventually turned out to be wrong, they marked significant steps in scientific reasoning. The first was the notion that there were lodestone mountains at the Earth's poles. Since lodestone was the only material Guinicelli knew that attracted a compass needle, he imagined there must be an enormous mass of it at the spot on Earth towards which all compasses were known to point – directly beneath the Pole Star. The notion of gigantic magnetic mountains at the poles spawned fantastic legends: apparently the mountains could even pull iron nails from passing ships.
Also captured in Guinicelli's verse was the idea that magnetic attraction, or 'virtue', was somehow transported through the air between the lodestone and the compass needle. Was this merely fanciful poetic language or an early glimmer of the concept of magnetic fields?
Just a few years later the whole way in which men studied nature was to take a new turn, thanks to a little-known Frenchman and his investigation of magnets. Pierre Pèlerin de Maricourt is believed to have been a knight and a crusader. Commonly known as Petrus Peregrinus, or sometimes as Peter the Wanderer, he was also a military engineer, well educated and something of a scholar. In 1269, while serving in the army of Charles d' Anjou at the siege of Lucera in southern Italy, Peregrinus had found time to reflect on and write about experiments he had earlier carried out. The result was Epistola de Magnete (Letter on the Magnet), dated August 8, 1269. Addressed to Sygerus de Foucaucourt, Peregrinus's neighbour in Picardy, Epistola de Magnete has been lauded as Europe's first work of true science.
Peregrinus had introduced the one crucial element missing from previous scientific endeavours: the idea of learning from experimentation. A contemporary, Friar Roger Bacon, himself a progressive and vigorous proponent of experimental science – he devised methods of making gunpowder, spectacles, mechanical flying-machines, ships, carriages and much more – would describe him as a master of experiment and one of only two perfect mathematicians, the other being one of Bacon's own students.
(Continues...)
Excerpted from North Pole South Pole by Gillian Turner. Copyright © 2010 Gillian Turner. Excerpted by permission of Awa Press.
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