Читать книгу Science: A History in 100 Experiments - John Gribbin - Страница 25

No. 20

TWITCHING FROGS AND ELECTRIC PILES

During the 1790s, a series of experiments led to two major discoveries: that electricity can flow from one place to another, and that electricity is important in operating the muscles of living animals. The second discovery came first, when the Italian physician Luigi Galvani was dissecting a frog. Galvani was also interested in the nature of electricity, and had in his laboratory a hand-cranked machine that could generate electric sparks by the friction of two surfaces rubbing together. This kind of ‘static’ electricity had been known about since the time of the Ancient Greeks. While Galvani was dissecting a pair of frog’s legs, a metal scalpel that had been in contact with the machine and had become electrified touched the sciatic nerve of one of the legs. The leg kicked as if it were still alive.

Galvani carried out many experiments to investigate the phenomenon. He found that legs from a dead frog would twitch if they were connected directly to the electric machine, or if they were laid out on a metal surface during a thunderstorm. But his most important discovery was a result of an observation, rather than a planned experiment.

© Science Photo Library

Luigi Galvani’s 1791 experiment on the legs of a frog. The upper diagram shows a silver rod (left) and a brass rod (right) being placed in contact with a foot and the spine of the frog. Bringing the two rods together resulted in the leg twitching as the muscles contracted. The lower diagram shows the metal rod connecting foils of two different metals, with the same result.

When preparing frogs’ legs for study, he would hang them up on brass hooks to dry out. When the hooks came into contact with an iron fence, the legs twitched. In case this was due to some influence from electricity in the air, Galvani took the legs and hooks indoors, away from any source of electricity (including his electrostatic generator) and brought the hooks into contact with iron again. Again, the legs twitched. He concluded that electricity must be manufactured in the body, and stored in the muscles of the frog. He called this ‘animal electricity’, and proposed that a fluid manufactured in the brain carries this electricity through the nerves of the body to its muscles. But he believed that this animal electricity was something different from the natural electricity of lightning, or the electricity produced artificially through friction.

Most of Galvani’s colleagues went along with this idea, which reinforced the idea of a special ‘life force’, or spirit, which distinguished living things from the non-living world. But one person in particular strongly disagreed. He was another Italian, a physicist called Alessandro Volta. Volta said that electricity was indeed the cause of the twitching of the legs of the dead frogs, but that it had not been stored in the muscles, and that there was no difference between animal electricity and natural electricity. Instead, he suggested that it was being generated from an outside source, an interaction between the two metals, brass and iron, that were in contact with one another.

Volta had already done a lot of work with electricity, including designing and building better friction machines to generate electric charge, and a device to measure electric charge. He first tested his new idea by putting different kinds of metal in contact with one another and touching the join with his tongue, which tingled as electricity flowed across the join. He realized that the saliva in his mouth was contributing to the effect, and in order to magnify the tiny current he felt with his tongue into something more dramatic he developed a new device, which he described in a letter to the Royal Society in 1800, two years after Galvani had died.

© Science Source/Science Photo Library

A drawing made by Alessandro Volta (1745–1827) of the first electric battery, called the ‘voltaic pile’.

Volta’s invention, developed during a long series of experiments, was literally a pile of alternating silver and zinc disks, separated from one another by cardboard discs soaked in brine. When the top disc in the pile was connected to the bottom disc by a wire, electric current flowed through the wire. When there was no connection, no current flowed. The device became known as a ‘voltaic pile’, the forerunner of the modern battery (the term ‘battery’ had actually already been used, by Benjamin Franklin, to describe a row of charged glass plates, which he likened to a row of cannon). And it had been developed specifically to disprove Galvani’s idea that electric current was part of a life force associated only with living things.

But Galvani and Volta were each partly right. Electricity is generated in the human body, as Galvani thought, but by chemical processes operating in living cells, not solely in the brain. On the other hand, there is nothing special about this electricity, which is exactly the same as the electricity generated in non-living systems such as voltaic piles.

After 1800, scientists could work with electric currents that they could turn on and off as they wished, and they could make the current stronger by adding more discs to the pile, or weaker by taking discs away. Very soon, scientists such as Humphry Davy (see here) were using this invention to revolutionize chemistry; one of the first discoveries was that passing an electric current through water decomposes the water into oxygen and hydrogen.