The Spark of Life: Electricity in the Human Body Read online

  The science of static electricity starts with the ancient Greeks’ fascination with amber. It is from their word for amber, electrum, which derives from elector, meaning ‘the shining one’, that we get the word electron, and hence electricity. Because it was usually found washed up on the seashore, amber’s origin was always considered mysterious. The historian Demostratus supposed it the crystallized urine of lynxes. Ovid tells a different story. He relates how Phaethon drove Apollo’s chariot (the Sun) too close to the Earth and was struck down by Zeus to prevent a catastrophe. His grief-stricken sisters were transformed into poplar trees and shed golden tears of amber that fell into the River Eridanus in which Phaethon drowned.

  Of course, we now know that amber is the petrified resin of extinct pine trees and are familiar with it as jewellery, or as the medium in which prehistoric insects have been encapsulated and perfectly preserved. But amber has another interesting and curious property. When rubbed with wool it generates static electricity, causing it to attract light, dry objects like small bits of tissue paper, feathers, specks of wheat chaff, and even your hair; this may be why Syrian women, who used decorative amber weights on the end of their spindles when spinning wool, called it the ‘clutcher’. Thales of Miletus is credited with being the first to note the attractive properties of amber, in the fifth century BC, although it is hard to be certain, as his findings were only passed down orally until later philosophers, such as Theophrastus, wrote them down.

  Amber generates a static charge because it attracts electrons from the atoms of the wool, becoming negatively charged in the process and leaving the wool positively charged. The charge is transferred by close contact between the amber and wool – the friction produced by rubbing is not involved, it is simply that rubbing greatly increases the area of contact between the two surfaces. Because opposite charges attract, any material that is naturally positively charged will leap towards the negatively charged amber. Conversely, as similar charges oppose one another, charging up your hair will cause each strand to repel its neighbours as much as possible, producing flyaway hair that stands on end like that of Shock-headed Peter in the German children’s picture book. Parenthetically, there is nothing static about ‘static’ electricity. The term refers only to the fact that the positive and negative electric charges are physically separated. As soon as a positively charged material comes close enough to a negatively charged one, current will flow from one to the other – as visibly demonstrated by the leap of an electric spark.

  It was William Gilbert, physician to Queen Elizabeth I, who first invented a sensitive instrument for measuring static electricity (an early electroscope). He used it to compile a long list of materials that could be electrified by rubbing. He also distinguished the attractive power of amber from that of magnets, arguing that two different phenomena are involved. Gilbert was a true scientist, for he advocated that you should not believe what you read, but instead try the experiment for yourself. He wrote, ‘Our own age has produced many books about hidden, abstruse, and occult causes and wonders, in all of which amber and jet are set forth as enticing chaff; but they treat the subject in words alone, without finding any reasons or proofs from experiments, their very statements obscuring the thing in a greater fog’. Hence, he concluded, ‘all their philosophy bears no fruit’. His words were prescient – present-day scientists make similar complaints about the advocates of astrology and alternative medicine.

  Great Balls of Fire

  The first machine capable of generating static electricity was invented by the German Otto von Guericke around 1663. It consisted of a giant ball of brimstone, about the size of a child’s head, with a wooden rod through its centre. The rod rested in a cradle, enabling the ball to be rotated on its axis by cranking the handle. When a dry hand or a pad of material was held against the whirling ball, static electricity was generated. It is unlikely that von Guericke appreciated that his machine produced electricity, in the modern sense of the word, but he did observe that the globe attracted feathers and other light material, and that once they had first touched the ball, the feathers were repelled and could be chased around the room by lifting the ball from its rod. Careful manipulation even allowed him to balance the feather on another object, such as a colleague’s nose.

  Frontispiece to Novi profectus in historia electricitatis, post obitum auctoris, by Christian August Hausen (1743), depicting the ‘flying boy’ experiment of Stephen Gray. Von Guericke’s ball can be seen on the right. The small boy on the left appears to be standing on an insulating drum, so he will not feel a shock when he touches the flying boy. However, when the gentleman does so, sparks will fly as the current leaps between their fingers and flows through his body to the ground.

  One of the most famous uses of von Guericke’s machine was the ‘flying boy’ experiment carried out by Stephen Gray in 1730, for which he was awarded the first Copley medal of the Royal Society of London. The child was suspended in mid-air by insulating cords of silk and then charged up by holding his feet against a rotating sulphur ball. Tissue paper, chaff and other light objects were attracted to his hands, and sparks flew from them when he was discharged.

  Large balls of sulphur are not easy to come by, so later electrostatic generators incorporated a circular plate (or spherical globe) of glass that rotated against a fixed cloth; one that was 50 inches in diameter was made for the Emperor Napoleon. The modern equivalent is the Van de Graaf generator, which can produce millions of volts and is well known for its use in spectacular ‘hair-raising’ demonstrations.

  A Jarring Shock

  There was no way to store static electricity until the invention of the Leyden jar in October 1745 by a German cleric, Ewald Jürgen von Kleist. Just a few months later the Dutch scientist Pieter van Musschenbroek reported a similar, independent, discovery to the Paris Academy of Sciences. His letter was translated by Jean-Antoine Nollet, the Abbot of the Grand Convent of the Carthusians in Paris, who named the device the Leyden jar, after Leiden, the Netherlands city in which Musschenbroek worked.

  The Leyden jar resembles an empty glass jam jar coated inside and out with a thin layer of metal foil that extends about two-thirds of the way up its sides. A brass rod is inserted through an insulating cork stopper into the neck of the jar and connected to the inner metal foil by a chain. If the outer layer of foil is grounded, the inside can be charged up by connecting the rod to a static electricity generator. This happens because the glass wall of the jar acts as an insulator and prevents the charge from flowing to the outer layer of foil, so that a very high charge difference can be built up between the two metal layers. The device is discharged by connecting the inner and outer layers of foil, either with two wires – which generates impressive electric sparks as the wires approach one another – or, more inadvisably, with one’s hands.

  The charge stored in a Leyden jar can be considerable – and extremely dangerous – as van Musschenbroek discovered. He wrote, ‘my right hand was struck with such force that my whole body quivered just like someone hit by lightning [. . .] the arm and the entire body are affected so terribly I can’t describe it. I thought I was done for.’ He also said that he would not repeat the experiment if offered the whole kingdom of France and cautioned others not to try it. But of course they did, with predictable effects. Some had convulsions or were temporarily paralysed. One German professor who got a severe shock and a bloody nose refused to test it on himself again and instead next tried it on his wife!

  The effects were clearly well known to Jules Verne, who described a fantastical device in Twenty Thousand Leagues under the Sea. In the novel, Captain Nemo explains to Monsieur Arronax that his underwater rifle fires glass capsules, which are ‘exactly like miniature Leyden jars, into which the electricity has been forced at a very high voltage. They discharge at the slightest impact, and however powerful the animal, it falls down dead’. His account contains some artistic licence, but shows how dangerous the Leyden jar was considered to be.

  The
severity of the shock from a Leyden jar surprised the experimenters because it was much stronger than that of a single spark produced by an electrostatic generator. This was because the jar could accumulate and store the charge flowing in many sparks, which would then be released all at once. Initially, it was believed that electricity was a fluid, so it seemed natural to use bottles and jars to store it in, but it was later appreciated that this was not the case and today the Leyden jar has been replaced by the capacitor. This operates on the same principle. It consists of two parallel metal plates separated by a thin layer of a non-conductive material such as mica, glass or air. The amount of charge a capacitor can store is determined by the area of the plates and the distance between them, and it can be considerable. The first atom smasher, built in the 1930s at Cambridge University by John Cockcroft and Ernest Walton, used banks of capacitors to generate and store up to almost a million volts.

  Nine Lords a-Leaping

  Another early demonstration of the effects of electricity on the human body was that conducted by the Abbé Nollet. In 1746, he ordered 200 of his monks to form up in a large circle almost a mile in circumference, holding long iron rods between their hands. Once they were all in position, the Abbé surreptitiously connected the two ends of the circle to a Leyden jar. The results were spectacular because the discharge of the jar sent a shock wave through the circle that caused all the monks to jump in turn, thereby demonstrating that electricity travels extremely fast. The French Academician Le Monnier wrote, ‘it is singular to see the multitude of different gestures and to hear the instantaneous exclamations of those surprised by the shock’. Hearing of the performance, King Louis XV demanded a rerun at Versailles and a company of 180 soldiers holding hands leapt simultaneously. Adam Walker, a popular British electrical performer in the late eighteenth century, went even further, boasting, ‘I have electrified two regiments of soldiers, consisting of eighteen hundred men.’

  Such experiments created a sensation. Public demonstrations of electrical phenomena rapidly became the rage, and itinerant lecturers roamed the country. Their aim was more spectacle than science, and performances were generally advertised for their entertainment qualities as much as their educational content. One of the most famous presenters was Benjamin Martin, a consummate entertainer who introduced a season of lectures at Bath in 1746 in which he used a Leyden jar to produce luminous discharges, and ‘wonderful Streams of Purple Fire’, which looked both beautiful and exotic in a darkened room. Like the Abbé Nollet, he also excited his audience by getting them to join hands and then applying an electric shock, which was not ‘so violent and dangerous as they have been represented, tho’ they are nearly as great as any Person (especially the Men) care to endure’. One letter of the time commented that these public spectacles were ‘the universal topic of discourse. The fine ladies forget their cards and scandal to talk of the effects of electricity’.

  On other occasions members of the public were invited to be charged up with static electricity and then ignite brandy or ether with sparks from their fingertips. Ladies donned glass slippers to insulate them from the ground and were electrified so that when their gentlemen friends approached with puckered lips outstretched, sparks flew between their lips. The electrified Venus, as she was known, gave stinging kisses. Electric toys abounded. Hidden words were magically revealed using ‘fulminating boards’ in which sparks jumped between small gaps in a conducting track, paper dancers were animated by the attractive and repulsive forces of static electricity, thunder houses were used to demonstrate the effects of lightning on buildings. Even more dramatic were the pistols and toy cannons that were fired using the heat of an electric spark.

  Many of these early demonstrations – and their protagonists – were viewed with suspicion because it was believed that electricity was a manifestation of the force of life and to tamper with it was blasphemy. For others it was a form of fire, which is why Mary Shelley subtitled her book The Modern Prometheus, after Prometheus, the mythical Greek who stole fire from the gods to give to mortals.2 At best, electricity was considered simply a novelty, an entertaining curiosity of no practical value. At which point Benjamin Franklin entered the scene and changed that view forever. In his hands, electricity left the salon and become the province of science.

  Snatching Lightning from the Sky3

  Franklin is widely believed to have been the first to show that lightning is a form of electricity. His most famous experiment was carried out in June 1752, when he flew a kite as a thunderstorm approached to prove that lightning is a stream of electrified air. He connected a short, stiff, pointed wire to the top of the kite, tied a metal key to the end of the kite string and attached the key to a silk ribbon, so insulating it from the ground. Whenever a thundercloud passed overhead, Franklin observed that the loose fibres of the hemp string would stand on end, suggesting that the twine became electrified. He even noted that a stream of sparks would leap from the key to his fingers, and that it was possible to charge a Leyden jar by touching it to the key. Franklin was fortunate not to have been struck by lightning, as this was a very dangerous experiment.

  But Franklin was not the first to demonstrate that lightning is an electrical discharge. That accolade goes to a Frenchman, Thomas-François Dalibard. In May of the same year, Dalibard erected an inch-thick, 40-foot-high iron pole, carefully insulating it from the ground by standing it on a plank balanced on three wine bottles and securing it with silken ropes. Sparks could be drawn from the rod with a Leyden jar when lightning was in the area. As Dalibard acknowledged, his experiment was inspired by Franklin’s paper describing his ‘Experiments and Observations’ on electricity, in which the American conjectured that such a pointed rod should draw lightning from the cloud and advised on how harm to the experimenter might be avoided. Dalibard’s demonstration created a sensation throughout Europe and was rapidly repeated by many others. Alas, not all were as careful or as lucky as Dalibard. The Swedish scientist Georg Wilhelm Richman was electrocuted a year later while experimenting with lightning conductors; his death is commemorated in a rather flowery poem by Erasmus Darwin (uncle of the more famous Charles), whose narrator –

  eyed with fond amaze

  The silver streams, and watch’d the sapphire blaze;

  Then burst the steel, the dart electric sped

  And the bold sage lay number’d with the dead!

  The Franklin Memorial in Philadelphia is inscribed with some of the statesman-scientist’s words of wisdom: ‘If you would not be forgotten as soon as you are dead and rotten, either write things worth reading or do things worth the writing.’ Franklin, of course, did both. One of his lasting legacies is the lightning conductor. Being aware that lightning was simply a form of electricity, and knowing that lightning strikes the tallest objects, he advised fixing on the ‘highest Parts of those Edifices upright Rods of Iron, made as sharp as a Needle and gilt to prevent Rusting, and from the Foot of those Rods a Wire down the outside of the Building into the Ground’. These pointed rods, he surmised, would conduct the strike safely to the ground so the building would not be damaged – or as he more poetically phrased it, ‘secure us from that most sudden and terrible Mischief!’

  Initially support for Franklin’s idea was not universal. Some objected that it would attract lightning to the house, thereby increasing the danger. Others considered it presumptuous as it interfered with the will of God; in Franklin’s time many people believed lightning was God’s punishment upon the sinful. Franklin countered that lightning was ‘no more supernatural than the Rain, Hail or Sunshine of Heaven, against the Inconveniences of which we guard by Roofs and Shades without Scruple’. His argument, and the manifest value of his invention, soon led to the installation of lightning conductors on most gunpowder stores, and even cathedrals.

  But in England there were problems. An acrimonious debate broke out between those who supported Franklin’s idea of a pointed tip to a lightning conductor and those who preferred a round knob, on the grounds
that a sharpened point was dangerously effective and attracted the lightning to it. The latter idea was championed by Benjamin Wilson. He campaigned vigorously against Franklin and he had powerful friends. Matters came to a head in 1777, when the gunpowder magazine administered by the Ordnance Board at Purfleet on the Thames was struck by lightning, and a few bricks were dislodged. The pointed rods previously installed on the advice of Franklin and his colleagues had seemingly not protected the building. Wilson took full advantage of the disaster, producing a spectacular electrical extravaganza at the Pantheon designed to prove that high spikes were dangerous and low blunted knobs were to be preferred. It was performed in the presence of King George III and many prominent ministers, who were impressed by his arguments. The fact that this took place at the time of the American War of Independence added a further charge to the issue. What had begun as a scientific spat quickly escalated into a major feud between the British knob and the American spike factions, with Wilson proclaiming that it was Britain’s patriotic duty to dismiss the invention of the enemy. Franklin’s friends countered with equally damaging political attacks. The Royal Society waded in, carried out a series of experiments and concluded that Franklin was correct. King George, however, sided with Wilson, ordering pointed spikes to be removed from all royal palaces and Ordnance buildings and demanding the Society reverse its conclusions. But John Pringle, the President of the Society, declined to do so, memorably stating that ‘duty as well as inclination would always induce him to execute his Majesty’s wishes to the utmost of his power; but “Sire [. . .] I cannot reverse the laws and operations of nature”’. The king promptly suggested he had better resign. Shortly after, a witty friend of Franklin’s lampooned the king in the following epigram: