Bright Sparks

A Little History Of Science: Bright Sparks

Have you ever wondered exactly what a flash of lightning is, and why a rumble of thunder follows? Violent displays of thunder and lightning happen high up in the sky, and are pretty dramatic, even if you know what causes them. Just as bolts of lightning always seek earth, by the early eighteenth century scientists had started to puzzle over this and about electricity much closer to home.

Another puzzle was over what came to be known as magnetism.

The ancient Greeks knew that if you rub amber (a yellowish semi- precious stone) very hard, it attracts small nearby objects to it. The cause of this power was difficult to understand. It seemed different from the constant power of a different kind of stone – the lode- stone – to attract objects containing iron. Just as a lodestar is a star that shows the way (especially the Northern Star), so the lodestone also guided travellers: it was a piece of a special mineral that, if suspended so that it could swing freely, would always point towards the magnetic poles. Lodestones could also be used to magnetise needles, and by the time of Copernicus, in the mid-sixteenth century, crude compasses were being used by seamen to help find their direction, since one end of the moveable needle of the compass always pointed to the north. An English doctor named William Gilbert wrote about this in 1600, when the word ‘magnetism’ had arrived. Both electricity and magnetism could produce entertaining effects and were popular topics for scientific lectures as well as after-dinner games.

Soon, people obtained even more powerful effects by rotating a glass globe on a point and rubbing it as it turned. You could feel and even hear the sparks as they were produced on the glass. This device became the basis of what was called the Leyden Jar, named after the town in the Netherlands where it was invented, in about 1745, by a professor at the university. The jar was half filled with water and connected to an electricity-generating machine by a wire. The connecting piece was called the ‘conductor’ because it allowed the mysterious power to pass into the water in the jar, where it was stored. (‘To conduct’ means ‘to lead’.) When a laboratory assistant touched the side of the jar and the conducting piece, he got such a jolt that he thought it was all over for him. The report of this experiment caused a sensation and Leyden Jars became all the rage. Ten monks once linked hands and when the first one touched the jar and conducting piece, they were all jolted simultaneously. An electric shock, it seemed, could be passed from person to person.

What exactly was going on? Beyond the games, there were serious scientific issues at stake. There were a lot of theories flying about, but one man who brought some order to the subject was Benjamin Franklin (1706–90). You might know him as an early American patriot who helped write the Declaration of Independence (1776) after the United States successfully achieved its independence from the British Empire. He was a witty, popular man, full of homespun wisdom, such as ‘Time is money’, and ‘In this world, nothing can be said to be certain, except death and taxes’. Next time you sit in a rocking chair, or see someone wearing bifocal glasses, think of him: he invented them both.

Largely self-taught, Franklin knew a lot about a lot of things, including science. He felt equally at home in France, Britain and America, and he was in France when he performed his most famous scientific experiment, with lightning. Like many people in the 1740s and 1750s, Franklin was curious about Leyden Jars and what they could show. In his hands, they showed far more than had been thought. First, he realised that things could carry either positive or negative charges – as you can see marked by the ‘+’ and ‘–’ at the opposite ends of a battery. In the Leyden Jar, the connecting wire and the water inside the jar were ‘electrised positively, or plus’, he said, while the outer surface was negative. The positive and negative were the same strength and so cancelled each other out.

Further experiments convinced him that the Jar’s actual power lay in the glass, and he made a kind of battery (he invented the word) by placing a piece of glass between two strips of lead. When he connected his device to a source of electricity, this ‘battery’ could be discharged of its electricity. Unfortunately, he did not pursue this discovery any further.

Franklin was not the first to puzzle about the relationship between the sparks generated by machines on earth and sparks in the sky, that is, lightning. But he was the first to apply what he had learned about the Leyden Jar to try to see how they might be connected. He devised a clever (but dangerous) experiment. He argued that electricity in the atmosphere would collect on the edge of clouds, just as it did in the Leyden Jar. If two clouds collided with each other, as they rolled across the sky during a thunderstorm, there would be a discharge of electricity – a flash of lightning. By flying a kite during such a storm, he could show that his idea was correct. The person flying the kite needed to be properly insulated from the electricity (by using a wax handle to hold the kite-string) and ‘grounded’ (with a piece of wire attached to him and trailing on the ground). Without these precautions, the shock of the electricity might kill him, and indeed, one unfortunate experimenter did die because he didn’t follow Franklin’s instructions. The kite experiment convinced Franklin that the electricity of lightning was like the electricity of Leyden Jars.

First gravity, now electricity: things in the heavens and on earth were being brought ever closer together.

Franklin’s work on electricity had immediate practical consequences. He showed that a metal pole with a sharp point conducted electricity to the ground. So, if such a pole were placed on the top of a building, with an insulated conducting body leading from it all the way down to the earth, lightning would be conducted away from the building, and it would not catch fire if struck by the lightning. This was a serious problem when houses were built mostly of wood and sometimes had thatched roofs. Lightning rods, as they are still called, act on this principle, and even now we use the word ‘earth’ for the bit of insulated wire in our electric plugs that takes away excessive electrical charge in things like washing machines and refrigerators. Franklin connected a lightning rod on his own house, and the idea caught on. There were important results to be had from understanding electricity.

The study of electricity was one of the most exciting areas of scientific research in the eighteenth century, and many ‘electricians’, as they were called, contributed to what we know today.

Three in particular have left their names with us. The first was Luigi Galvani (1737–98), a doctor who liked to tinker with electrical apparatus and animals. He practised medicine and taught both anatomy and obstetrics (the medical management of child- birth) at the University of Bologna, but he was also much interested in physiological studies. While investigating the relation between muscles and nerves, he discovered that a frog’s muscle could be made to contract if the nerve attached to it were connected to a source of electricity. After further research, he likened muscle to a Leyden Jar, able to generate and discharge a current of electricity. Electricity was an important part of animals, Galvani said.

Indeed, ‘animal electricity’, as he termed it, seemed to him to be an essential ingredient of how animals functioned. And he was right.

Static electric shocks, which occur when electricity that has built up on the surface of an object is discharged, are still called galvanic shocks. Scientists and electricians use galvanometers to measure electric currents. Galvani’s notion of animal electricity attracted a good deal of criticism, especially from Alessandro Volta (1757– 1827), a scientist from Como in northern Italy. Volta had a low opinion of doctors who branched out into physics, and he set out to show that animal electricity did not exist. Volta and Galvani had a very public debate about the interpretation of Galvani’s experiments. In the course of his extensive work aimed at discrediting Galvani, Volta examined the electric eel, which, as could be demonstrated, did produce electricity. He believed that even these animals did not make Galvani’s ‘animal electricity’ more convincing.

More importantly, Volta discovered that if he built up successive layers of zinc and sliver, and separated them by layers of wet card- board, he could produce a continuous electric current through all the layers. Volta sent news of his invention, which he called a ‘pile’, to the Royal Society in London. Like the Leyden Jar, it created a sensation in England and France.

At this time, France was busy conquering northern Italy, and the French Emperor, Napoleon Bonaparte, decorated the Italian physicist for his invention, for it offered a reliable source of electric currents for experimental research. Volta’s ‘pile’ went on to play an essential part in early nineteenth-century chemistry. It was the practical development of Franklin’s ‘battery’, and has become essential in our lives today. We remember Volta because his name gave us the word ‘volt’ which is one way we measure electrical power – check out the packaging next time you change a battery.

Our third great electrician (and formidable mathematician) also gave his name to the measurement of electricity: André-Marie Ampère (1775–1836). We get the word ‘amp’ from his name.

Ampère lived through the trauma of the French Revolution and its aftermath, during which his father lost his head on the guillotine.

His personal life was equally sad. His beloved first wife died after the birth of their third child, and his second marriage was deeply unhappy and ended in divorce. His children turned out badly, and he was constantly beset with money worries. In the middle of this chaos, Ampère realised some fundamental things about mathematics, chemistry and, above all, what he called ‘electrodynamics’. This complicated subject brought together electricity and magnetism. Despite its complexity, Ampère’s simple but elegant experiments showed that magnetism was in fact electricity in motion. His work underpinned that of Faraday and Maxwell, and so we will talk about it in more detail when we come to these later giants of electromagnetism. Although later scientists showed that many of the details of Ampère’s theories did not lead anywhere, he provided the starting point for much research into electromagnetism. It is important to remember that science is also about sometimes getting things wrong.

By the time of Ampère’s death, electricity had gone a long way towards being tamed. Franklin’s work had been homespun and, important though it was, he was an ingenious amateur compared with Galvani, Volta and Ampère, who used more sophisticated equipment, and worked in laboratories. Galvani had the last laugh on Volta, for we now know that electricity plays an important part when muscles and nerves interact.