by Brian Clegg
††† Surely the highlight of Maxwell’s equipment, this was an impressive mahogany-framed six-cell battery retailing at an expensive £3 10 shillings. The advertising for the apparatus claimed that it would ‘heat to redness 4 inches of platinum wire, fuse iron wire with facility, and empower a sufficiently strong electro-magnet to sustain many hundredweights’.
‡‡‡ Not to be tried at home as the glass may well shatter. As was not unusual at the time, Maxwell seems to have had little concern for personal health and safety.
§§§ I.e. the University of Cambridge, traditionally given the abbreviation ‘Cantab’ from Universitas Cantabrigiensis, its Latin name.
Demonic Interlude II
In which electricity meets magnetism
With my creator safely set off on his way to Cambridge, this is a good opportunity to give you a little background on a topic that would come to dominate much of Jimmy’s* life – electromagnetism. While inevitably from my own viewpoint it’s JCM’s interest in thermodynamics and the related field of statistical mechanics that makes him of particular value, a truly objective observer† would probably have to assess his contributions to electromagnetism as having the greater impact on the world.
Both electricity and magnetism were known as strange natural phenomena long before there was any idea of what might be going on, each considered totally separate from the other.‡ We can get an idea of where the concept of electricity originated from its name. Both ‘electricity’ and ‘electron’ are derived from electrum, the Latin for amber (a name itself derived from a similar Greek word).
Natural electricity
When amber is rubbed, it generates static electricity in the same way that a balloon gets charged up when it is rubbed on your hair. This so-called triboelectric effect involves electrons coming loose as a result of the rubbing process, leaving the object rubbed and what it’s rubbed with each having the opposite electrical charge. Electrical attraction means the balloon can then pick up very light objects through an induced charge and it can even produce a tiny spark. Induction comes into the electrical business quite frequently – it simply means that if you bring something that’s electrically charged near an object it will tend to repel particles with the same charge in that object, so that the nearest side of the object has an opposite electrical charge, making the object – in this case, perhaps, a small piece of paper – attracted to the original source.
The triboelectric effect is probably also the mechanism responsible on a much larger scale for the charge build-up in the most dramatic natural example of electricity, a bolt of lightning. It’s worth spending a little time on lightning to get a feel for the early impression of electricity.
Lightning was surely the first observed electrical phenomenon, though initially its sheer magnitude resulted in it being labelled the work of an irritable god. Thanks to big-budget superhero movies, probably the best-known thunder god these days is the relative latecomer, the Norse god Thor; though earlier, the power of lightning-throwing was thought to be held by Zeus for the Greeks, Jupiter for the Romans, and Indra in the Hindu pantheon. No doubt there have been many more. When gods weren’t blamed, lightning, like other strange appearances in the sky such as comets and red moons, was considered to be a warning of dire events to come. In the first century AD, for example, Pliny the Elder said that a thunderstorm was prophetic, direful and cursed.
The superstitious view of lightning is hardly surprising. It’s the most dramatic natural phenomenon that the majority of people will experience – certainly the most common of nature’s big beasts, with at any one time typically around 1,800 thunderstorms active around the world. And lightning doesn’t just look and sound impressive, it has the capability to blast trees and to kill.
One common myth associated with lightning is that it does not strike the same place twice. Some rural areas of Britain used to have a brisk trade in what were known as thunderstones. These were stones that had a hole in the middle, which were bought as a safeguard to place up the chimney of a house. The idea was that the thunderstone had already been struck by lightning, which was thought to have caused the hole, so lightning would be unable to hit the chimney without breaking the ‘striking the same place twice’ rule.
Unfortunately, there are two problems with this folk remedy. One is that lightning is entirely happy to strike the same place twice and quite often does so. If a location is susceptible to lightning strikes, it’s not unusual for it to get several hits in one day. The Empire State Building, for instance, has received as many as fifteen strikes in a single storm. When you think about it, lightning would have to be conscious and directed if it never returned to the same spot.
Perhaps the most impressive example of multiple strikes, though, was not on a building but on the US park ranger Roy Sullivan, who entered the Guinness Book of Records as the person who had been hit by lightning the most times – a total of seven strikes, all of which he survived. The other problem with using thunderstones to ward off lightning is that these unusual formations weren’t caused by lightning at all – they are the remains of Stone Age hammers where a wooden handle and leather bindings have long ago rotted away.
In general demonic use (and we use them a lot), thunder and lightning are pretty much interchangeable because we now know that thunder is simply the noise produced by the lightning bolt ripping open the air – traditionally they were considered linked but independent events because of the variable time difference between them. This reflects light’s immense speed of 299,792,458 metres per second compared with sound’s plodding 343 metres per second at sea level.
It’s hard to imagine that something as dramatic as a lightning bolt could be down to a similar procedure to rubbing a balloon on your hair – and, to be honest, the means of lightning production isn’t anything like certain – but the model that has been generally supported since the 1950s is a triboelectric mechanism. In a thundercloud there are particles of ice and supercooled water droplets as well as graupel (miniature hailstones), all churning and jostling about in the clouds as warmer and cooler air streams collide. This is thought to result in the heavier graupel getting a relative negative charge. The charge becomes separated as the graupel sink towards the bottom of the cloud while the lighter, positively charged particles rise. This conveyor-like process is thought to occur many times, gradually building up a bigger and bigger electrical potential difference.
It has been suggested by some scientists that cosmic rays could also have a role to play in triggering lightning. Cosmic rays are streams of high-energy charged particles that come rocketing towards the Earth, but that are mostly fended off by the planet’s magnetic field and the atmosphere. Russian researchers have suggested that cosmic rays could produce a stream of electrons that build up in a chain reaction as the ice particles circulate. However, other scientists consider this mechanism highly doubtful.
From the skies to the laboratory
What we do know for certain, though, is that lightning is nothing more than an electrical discharge like that produced by rubbing a balloon – though admittedly a discharge with a remarkable kick. The eighteenth-century American journalist, diplomat and scientist Benjamin Franklin is famous for demonstrating this by flying a kite in a thunderstorm with a key attached to the kite string. Or rather he certainly described the experiment in 1750 – it’s quite possible, though, that he had someone else carry out the risky procedure.
It’s very unlikely that Franklin (or whoever he persuaded to take the risk) flew a kite and waited for it to be struck by lightning, as the legendary experiment is often portrayed. Instead, his proposal was to tap into the electrical charge in the thunderclouds to cause a build-up of electricity on the key by induction, with no lightning strike taking place. The charge from the key was then passed to a Leiden jar, an early method of storing electricity, where it could be demonstrated that the electricity from storm clouds was just the same as the tamer-seeming ground-based variety.
There’s no
doubt that some people have attempted the experiment Franklin described, but it is phenomenally dangerous. Just consider the amount of energy in a typical lightning flash – perhaps half a billion joules. It takes 10 joules to run a 10-watt bulb for a second. The amount of energy in a lightning bolt is more like the output of a mid-sized power station for a second. The electrical current rips through the air, heating it suddenly and causing the rumble of thunder. The temperature in a bolt peaks at around 20,000–30,000°C, as much as five times the surface temperature of the Sun.§
You can’t see electricity itself in a lightning bolt. It is atoms that have received energy from the lightning which blast out light as their electrons are boosted in energy then fall back to their usual levels. And what’s produced is the full spectrum of electromagnetic radiation, all the way from radio waves up to X-rays and gamma rays. We don’t expect electricity to be able to flow through the air because our atmosphere is quite a good insulator. It takes around 30,000 volts to get a spark to jump across just one centimetre at normal levels of humidity (the damper the air, the easier it is for electricity to flow).
It seems reasonable that dampness helps because we are used to water being seen as a good conductor of electricity (which is why it’s not a good idea to get electrical equipment wet). Oddly, though, like air, water is actually a reasonably good insulator. Take absolutely pure water and it hardly conducts at all. But it almost always contains the ions of substances that are dissolved in it, and these carry the current. Ions (electrically charged atoms) are also responsible for carrying the electricity through the air in lightning, but to produce the vast streamers of electric discharge in a bolt of lightning still takes a huge amount of electrical power.
Once a significant secondary charge has been induced, something weird happens. There is a relatively weak flow of electricity between the negative storm cloud and its positive target. This flow of electricity ionises the air. Just as in water, a collection of ions in the air conducts electricity much better than a collection of neutral atoms. This weak discharge from the cloud, called a leader, sets up a path for the main burst of lightning, the return stroke, which goes in the opposite direction – in the case of a ground strike, the main stroke runs from the ground up to the cloud rather than in the obvious direction.
Whether or not Benjamin Franklin took a kite out in a thunderstorm, he certainly did invent the lightning rod (also known as a lightning conductor). The idea of this simple device is to provide a metal spike on the highest part of a building, connected via a thick metal conductor to the ground. This spike is most likely to receive a hit and was intended to conduct the electricity away, reducing damage to the structure. In practice more often than not the lightning rod prevents a lightning strike from ever happening. The rod allows any charge being induced around the spike to leak away to the ground, reducing the chances of a leader forming.
Lightning gives a dramatic natural example of electricity in action, but it’s difficult to study because it does not take place in a controlled environment. By the eighteenth century, static electricity was being used in dramatic demonstrations such as the ‘electric boy’, where a youth was suspended from ribbons and charged up by rubbing glass rods with silk. He would then be used to give the audience shocks and to attract light objects. However, it would be the nineteenth century before usable current electricity, where electrical charges flow through wires, became viable. Before we get on to that, we need to take a step back and bring magnetism alongside its electrical cousin.
Magnetic matter
Magnets have been known since ancient times in the form of naturally occurring lodestones. These are chunks of the mineral magnetite – an oxide of iron. Most magnetite has no special properties, but when it carries certain impurities it has the right structure to become a permanent magnet. It’s not 100 per cent certain how the lodestones became magnetised in the first place – the most obvious suspect, the Earth’s magnetic field, is far too weak. It is suspected, particularly as lodestones only tend to be found near the surface of the Earth, that they were magnetised by lightning strikes (so, in effect, lodestones are the true thunderstones). With the kind of symmetry we demons prefer to avoid – as humans find it far too attractive – once we’ve united electricity and magnetism, each is capable of producing the other.
The earliest recorded attempt to take a scientific approach to magnetism came from a thirteenth-century French scholar named Peter de Maricourt, though better known as Peter Peregrinus (‘Peregrinus’ usually referred to a stranger or foreigner – the implication is that he was a wanderer or pilgrim who didn’t stay long at a single institution). We know little of Peter himself, though there was a village called Mehariscourt, near the abbey of Corbie in Picardy, the French region that he is associated with in legend. The thirteenth-century English natural philosopher and friar Roger Bacon said of Peter, whom he had met in Paris:
He gains knowledge of matters of nature, medicine and alchemy through experiment, and all that is in the heaven and in the earth beneath. He is ashamed if any old woman or soldier or countryman knows about something he does not know about the country. So he has pried into the work of metal-founders and what is wrought in gold and silver and other metals and all minerals …
So without him philosophy cannot be completed, nor yet handled with advantage and certainty. But just as he is too worthy for any reward, so he does not seek one for himself. For if he wished to dwell with kings and princes, he could easily find those who would honour and enrich him. Or if in Paris he were to display his knowledge through the works of wisdom, the whole world would follow him. But because in both these ways he would be hindered from the bulk of the experiments in which lie his chief delight, he neglects all honours and riches, especially since he will be able, when he wishes, to reach riches by his own wisdom.
In 1269 Peter completed his Epistola de Magnete, a detailed letter which describes the two differing poles of a magnet, attraction and repulsion, how to magnetise iron with a lodestone, and the Earth’s magnetism. He also gives some details on constructing compasses, both by the then common approach of floating a magnet on water and by using the more advanced approach of mounting a thin magnet on a pivot. Peter’s work describes practical applications rather than providing any theoretical basis, but it was the definitive document on the subject through to the start of the seventeenth century. Then, Peter’s work was eclipsed by that of the English natural philosopher William Gilbert.
Gilbert’s book, De Magnete, went into far more detail than Peter had been able to in his letter, and gave far greater consideration to how the Earth itself could act as a huge magnet. To explore the way that such a magnet would behave, Gilbert constructed spherical metal magnets known as terrella. These helped him understand the property of dip, where a compass needle does not point horizontally due to its position on the Earth’s surface.
Inevitably Gilbert did not get everything right. His biggest error was suggesting that gravity was the same as magnetism, though the similarity of principle was a good observation. But his book, which also covered some aspects of static electricity, restarted the interest in thinking about the nature of magnetism over and above its usefulness for constructing compasses.
The birth of electromagnetism
We’re now well placed to move on to the discovery of current electricity – electricity that flows from place to place – thanks to Italian physicist Alessandro Volta’s 1799 electrical pile (a collection of which was called a battery), and the Danish Hans Christian Oersted’s 1820 discovery that electrical currents produce a magnetic effect.
These were precursors to the remarkable work on electricity and magnetism that Michael Faraday would do. Faraday realised that the two were strongly interconnected and made popular the term ‘electromagnetism’, devised for their combined study by Oersted (though Faraday wrote it, as would be common initially in English, as electro-magnetism). Faraday’s discoveries proved essential for the work that JCM would carry out.
Because of Faraday’s importance to the development of JCM’s thinking, his role needs exploring further. Faraday’s family had come to London from Westmorland, in the English Lake District, before he was born in search of work for his blacksmith father. In 1805, at the age of fourteen, Faraday was apprenticed to bookbinder George Riebau, a refugee from the French revolution. Faraday spent all his spare time in the shop, teaching himself from the books that were in to be bound. These heavy volumes, along with the lectures provided by a self-improvement group, the City Philosophical Society, gave Faraday a single-minded intent to break into the closed world of science.
One of Riebau’s clients, a Mr Dance, got Faraday a temporary job assisting the Royal Institution’s star scientist, Humphry Davy, while Davy’s usual assistant was injured. The Institution was a relatively new organisation, established in 1799 by leading British natural philosophers as a means to spread the word about science to the wider population and to provide facilities for undertaking research. Working with Davy was a dream opportunity for Faraday, but he was soon sent back to the bookbinders. Faraday kept up a steady barrage of applications for jobs in scientific establishments. Eventually, in 1813, he got the permanent post of lab assistant at the Royal Institution.
By 1821, with a promotion under his belt, the young scientist was making steady, if unremarkable progress. He was asked to write an article summarising the latest position on electromagnetism, the emerging field of the interaction between electricity and magnetism. To help do so, Faraday replicated the experiments he had read about. As he passed electricity down a wire running alongside a fixed magnet, he saw something that first puzzled him, then challenged his imagination. When the current flowed, the wire moved, circling round the magnet. As far as he could tell, this was a new discovery, and he realised that it needed publication. He wrote it up with little consultation with his peers and was promptly accused of plagiarism.