Franklin that storm clouds must be electrically charged, like a glass rod rubbed with silk, and that lightning occurred when this static electricity was discharged to the ground, like the sparks that flew when the rod came near another object. But how could this idea be tested? Franklin wrote about his experiments in letters he sent to Collinson in England, and suggested that a tall metal rod, or spike, might be erected during a thunderstorm to draw off electricity from the clouds. The idea was not to encourage lightning to strike the metal rod, but to draw the electricity off gently and capture it in a special kind of glass bottle, known as a Leyden (or Leiden) Jar. Apart from Collinson, nobody at the Royal Society was enthusiastic about Franklin’s ideas. But Collinson had them published in the early 1750s, and they were widely read by scientists in mainland Europe.
One of those scientists, Thomas-François d’Alibard, decided to put Franklin’s idea to the test. In May 1752 he set up a 40-foot (12-metre) high metal spike in a garden in Marly-la-Ville, in northern France, and drew off sparks from it during a storm. The rod was not, however, struck by lightning, which probably would have killed d’Alibard. Before news of this success could reach America, Franklin had carried out his own version of the experiment in Philadelphia, by flying a kite during a thunderstorm in June 1752.
Like d’Alibard, Franklin knew that it would be dangerous to be struck by lightning, and was simply trying to draw off some of the electric charge from the clouds, along the wet string of the kite to a key attached to the string. To encourage this, a pointed wire was attached to the kite itself, but as a safety precaution he held the kite by a silk ribbon, attached to the string of the kite, as an insulator. Sure enough, electricity was drawn down to the key, and, when an object was moved close to the key, sparks flew across the gap. Franklin even let the sparks leap across, painfully, to his own knuckles.
© Sheila Terry/Science Photo Library
French scientist Thomas-François d’Alibard (1709–1799) carrying out his lightning experiment on 10 May 1752, at Marly, France.
Soon after he had carried out his own experiment, Franklin heard of d’Alibard’s success in France. In October 1752, he wrote to Collinson with directions on how to repeat his own experiment: ‘When rain has wet the kite twine so that it can conduct the electric fire freely, you will find it streams out plentifully from the key at the approach of your knuckle, and with this key a phial, or Leiden jar, may be charged: and from electric fire thus obtained spirits may be kindled, and all other electric experiments [may be] performed which are usually done by the help of a rubber glass globe or tube; and therefore the sameness of the electrical matter with that of lightening completely demonstrated.’6
Other experimenters were not so careful – or rather, not so lucky – as Franklin, and several people were killed by lightning while trying to copy what he had done. In Franklin’s case, the electricity was drawn down gradually from the clouds, but he was wrong to think that lightning itself could not strike via the kite. The same flawed thinking was behind his invention of the lightning conductor, in the form of a metal rod attached to the highest point of a building and connected to the earth by a wire. He thought that such a rod would draw off electricity gradually, and prevent a lightning strike. In fact, such a lightning conductor encourages the lightning to strike, but protects the building by acting as a direct route to earth for the lightning, which strikes the metal rod rather than the building itself. But either way, it does work! And all of this proved that lightning is indeed the same phenomenon as static electricity, but on a larger scale.
| No. 15 | THE HEAT OF ICE |
Ice has an intriguing property, which fascinated scientists studying the nature of heat in the eighteenth century. As well as being intrinsically interesting, these studies had practical implications; it was just at the time steam power was beginning to be harnessed to drive the Industrial Revolution. The curious property is that when ice at the freezing point (0 ºC, or, in the units used in Britain then, 32 ºF) is heated, its temperature stays the same until all the ice has melted into water. Only then does the temperature of the water increase as more heat is applied. The same sort of thing, of course, happens when other substances, such as metals, are melted, but ice is much easier to study.
Other people had thought that if a lump of ice at the melting point were heated by a tiny amount it would all melt. But the person who studied what was really going on in a careful series of experiments in the 1760s was a professor at Glasgow University, Joseph Black. Whenever Black did experiments, he measured everything that could be measured, as accurately as possible. He had made his name by studying the amount of gas produced or absorbed in chemical reactions. In one of his experiments, a carefully weighed amount of limestone was heated, to produce quicklime, which was then weighed. The quicklime weighed less, because the gas we now call carbon dioxide had been driven off. A weighed amount of water was added to the quicklime to produce slaked lime, which was weighed. Then, a weighed amount of a mild alkali was added to convert the slaked lime back into what weighing proved to be the same amount of limestone that he had started with. Along the way, the differences in weight told him how much gas had been lost or absorbed at each stage. This was quantitative science, as opposed to qualitative science, in which the changes in the character of the substances (their quality) was noted, but there were no measurements of how much they had changed (the quantity).
© Middle Temple Library/Science Photo Library
Joseph Black (1728–1799).
© Sheila Terry/Science Photo Library
Joseph Black giving a practical demonstration of latent heat to students of Glasgow University in the 1760s.
Black carried over this quantitative approach – a cornerstone of modern experimental science – into his studies of heat. He found that the amount of heat needed to melt a certain amount of ice at 32 ºF into water at the same temperature was enough to raise the temperature of the water from 32 ºF all the way to 140 ºF (or 60º C). He also studied the way water turns into steam, showing that when a mixture of water and water vapour at the boiling point (212 ºF, or 100 ºC) is heated, the temperature does not increase until all the water has been turned into vapour. And if a certain weight of water – say, a pound – at 32 ºF is added to the same quantity of water at 212 ºF, the resulting liquid has a temperature of 122 ºF (or 50º C), halfway between boiling and freezing. This led him to the idea of ‘specific heat’, which is the amount of heat required to raise the temperature of a certain amount of stuff by one degree (in modern units, the heat required to raise the temperature of 1 gram by 1 ºC). Black coined the term ‘specific heat’, and also gave the name ‘latent heat’ to the heat absorbed by a melting substance. And when a liquid such as water freezes, the same amount of latent heat is released as it does so; similarly, latent heat is released when vapour condenses into liquid. In Black’s own words: ‘I, therefore, set seriously about making experiments, conformable to the suspicion that I entertained concerning the boiling of fluids … I imagined that, during the boiling, heat is absorbed by the water, and enters into the composition of the vapour produced from it, in the same manner as it is absorbed by ice in melting, and enters into the composition of the produced water. And, as the ostensible effect of the heat, in this last case, consists, not in warming the surrounding bodies, but in rendering the ice fluid; so, in the case of boiling, the heat absorbed does not warm surrounding bodies, but converts the water into vapour. In both cases, considered as the cause of warmth, we do not perceive its presence: it is concealed, or latent, and I give it the name of LATENT HEAT.’7
These discoveries were noted by a certain James Watt, an instrument maker at the university, who built experimental apparatus for Black, and who went on to develop steam engines.