THE BRIEFLY HISTORY OF PHYSICS
Gilbert and the amber force: 1600
The year 1600 is a good one for William Gilbert. He is appointed court physician to Queen Elizabeth, and the summary of his life-long research into magnetism is published as De magnete, magneticisque corporibus, et de magno magnete tellure (Of the magnet, of magnetic bodies, and of the earth as a great magnet).
As the title states, Gilbert's work has led him to the grand conclusion that compasses behave as they do because the earth itself is a vast magnet. He introduces the term 'magnetic pole', and states that the magnetic poles lie near the geographic poles.
Gilbert describes useful practical experiments, revealing how iron can be magnetized for use in compasses without relying on rare and expensive lodestone. Hammering the metal will do the trick, if the iron is correctly aligned with the earth's magnetic field.
Gilbert's researches also involve him in the mysterious property of amber, recognized 2000 years previously by Greek scientists. He identifies this as a force and coins a term for it from elektron, the Greek for amber. He calls it, in an invented Latin phrase, vis electrica - the 'amber force'. Electricity has found its name.
Galileo and the Discorsi: 1634-1638
In December 1633 Galileo is place under house arrest, on the pope's orders, because of his work on astronomy. Finding himself confined to his small estate at Arcetri near Florence, his response is typically positive. He settles down to explain and prove his early and less controversial discoveries in the mechanical sciences.
Two are particularly well known. The first he is said to have observed as a student in Pisa, when he watches a lamp swinging in the cathedral, times it by his own pulse, and discovers that each swing takes the same amount of time regardless of how far the lamp travels. At Arcetri he demonstrates this principle of the pendulum experimentally, and suggests its possible use in relation to clocks.
His other most famous discovery in physics, proved theoretically in about 1604 when he is professor of mathematics in Padua, is that bodies falling in a vacuum do so at the same speed and at a uniform rate of acceleration. (There is as yet no vacuum in which to demonstrate this law, but Boyle is able to do so later in the century.) While at Padua Galileo also works out the laws of ballistics, or the dynamics of objects moving through the air in a curve rather than falling directly to earth.
Written up and proved mathematically during 1634, these theorems are published in Leiden in 1638 as the Discorsi e dimostrazioni matematichè intorno à due nuove scienze attenenti alla mecanica et i movementi locali.
Galileo's title claims to introduce two new sciences, mechanics and 'local movements', and his book stands at the start of mathematical physics. He is the first to use mathematics to understand and explain physical phenomena, and he is the first to make rigorous use of experiment to check results provided by theory. The attractive notion of his dropping weights from the leaning tower of Pisa, to check on the behaviour of falling bodies, is only a legend. But he certainly, if more mundanely, rolls balls down inclined planes for the same purpose.
Galileo provides the foundation on which Newton (born in the year of Galileo's death) soon builds.
Barometer and atmospheric pressure: 1643-1646
Like many significant discoveries, the principle of the barometer is observed by accident. Evangelista Torricelli, assistant to Galileo at the end of his life, is interested in why it is more difficult to pump water from a well in which the water lies far below ground level. He suspects that the reason may be the weight of the extra column of air above the water, and he devises a way of testing this theory.
He fills a glass tube with mercury. Submerging it in a bath of mercury, and raising the sealed end to a vertical position, he finds that the mercury slips a little way down the tube. He reasons that the weight of air on the mercury in the bath is supporting the weight of the column of mercury in the tube.
If this is true, then the space in the glass tube above the mercury column must be a vacuum. This plunges him into instant controversy with traditionalists, wedded to the ancient theory - going as far back as Aristotle - that 'nature abhors a vacuum'. But it also encourages von Guericke, in the next decade, to develop the vacuum pump.
The concept of variable atmospheric pressure occurs to Torricelli when he notices, in 1643, that the height of his column of mercury sometimes varies slightly from its normal level, which is 760 mm above the mercury level in the bath. Observation suggests that these variations relate closely to changes in the weather. The barometer is born.
With the concept thus established that air has weight, Torricelli is able to predict that there must be less atmospheric pressure at higher altitudes. It is not hard to imagine an experiment which would test this, but the fame for proving the point in 1646 attaches to Blaise Pascal - though it is not even he who carries out the research.
Having a weak constitution, Pascal persuades his more robust brother-in-law to carry a barometer to different levels of the 4000-foot Puy de Dôme, near Clermont, and to take readings. The brother-in-law descends from the mountain with the welcome news that the readings were indeed different. Atmospheric pressure varies with altitude.
Von Guericke and the vacuum: 1654-1657
Spectators in the town square of Regensburg, on 8 May 1654, are treated to perhaps the most dramatic demonstration in the history of science. Otto von Guericke, burgomaster of Magdeburg and part-time experimenter in physics, is about to demonstrate the reality of a vacuum.
Aristotle declared that there can be no such thing as empty space, but von Guericke has spent several years perfecting an air pump which can achieve just that. He now produces two hollow metal hemispheres and places them loosely together. There is no locking device. Von Guericke works for a while at his pump, attached by a tube to one of the hemispheres. He then signals that he is ready.
Sixteen horses are harnessed in two teams of eight. Each team is attached to one of the hemispheres. Whipped in opposite directions, the horses fail to pull the sphere apart. Yet when von Guericke undoes a nozzle of some kind, the two halves separate easily.
A mysterious point has been very forcefully made. Von Guericke's experiments are first described in a book of 1657 (Mechanica Hydraulica-Pneumatica by Kaspar Schott). The vacuum thus becomes available to the scientific community as an experimental medium. Von Guericke himself uses it to demonstrate that a bell is muffled in a vacuum and a flame extinguished. Robert Boyle, too, soon borrows the device.
Robert Boyle: 1661-1666
The experimental methods of modern science are considerably advanced by the work of Robert Boyle during the 1660s. He is skilful at devising experiments to test theories, though an early success is merely a matter of using von Guericke's air pump to create a vacuum in which he can observe the behaviour of falling bodies. He is able to demonstrate the truth of Galileo's proposition that all objects will fall at the same speed in a vacuum.
But Boyle also uses the air pump to make significant discoveries of his own - most notably that reduction in pressure reduces the boiling temperature of a liquid (water boils at 100° at normal air pressure, but at only 46°C if the pressure is reduced to one tenth).
Boyle's best-known experiment involves a U-shaped glass tube open at one end. Air is trapped in the closed end by a column of mercury. Boyle can show that if the weight of mercury is doubled, the volume of air is halved. The conclusion is the principle known still in Britain and the USA as Boyle's Law - that pressure and volume are inversely proportional for a fixed mass of gas at a constant temperature.
Boyle's most famous work has a title perfectly expressing a correct scientific attitude. The Sceptical Chymist appears in 1661. Boyle is properly sceptical about contemporary theories on the nature of matter, which still derive mainly from the Greek theory of four elements.
His own notions are much closer to the truth. Indeed it is he who introduces the concept of the element in its modern sense, suggesting that such entities are 'primitive and simple, or perfectly unmingled bodies'. Elements, as he imagines them, are 'corpuscles' of different sorts and sizes which arrange themselves into compounds - the chemical substances familiar to our senses. Compounds, he argues, can be broken down into their constituent elements. Boyle's ideas in this field are further developed in his Origin of Forms and Qualities (1666).
Chemistry is Boyle's prime interest, but he also makes intelligent contributions in the field of pure physics.
In an important work of 1663, Experiments and Considerations Touching Colours, Boyle argues that colours have no intrinsic identity but are modifications in light reflected from different surfaces. (This is demonstrated within a few years by Newton in his work on the spectrum.)
As a man of his time, Boyle is as much interested in theology as science. It comes as a shock to read his requirements for the annual Boyle lecture which he founds in his will. Instead of discussing science, the lecturers are to prove the truth of Christianity against 'notorious infidels, viz., atheists, theists, pagans, Jews and Mahommedans'. The rules specifically forbid any mention of disagreement among Christian sects.
Newton in the garden: 1665-1666
The Great Plague of 1665 has one unexpected beneficial effect. It causes Cambridge university to close as a precaution, sending the students home. A not particularly distinguished member of Trinity College, who has recently failed an examination owing to his feeble geometry, travels home to the isolated Woolsthorpe Manor in Lincolnshire.
He spends there the greater part of eighteen months, one of the most productive periods in scientific history. With time for uninterrupted concentration, he works out the binomial theorem, differential and integral calculus, the relationship between light and colour and the concept of gravity. The student is the 22-year-old Isaac Newton.
The famous detail of the falling apple in the garden of Woolsthorpe Manor, as the moment of truth in relation to gravity, provides the perfect seed for a popular legend. But the story is first told in the next century, by Voltaire, who claims to have had it from Newton's step-niece. In reality it is the moon which prompts Newton's researches into gravity.
Meanwhile his discoveries in relation to light and colour bring him his first fame.
Newton and Opticks: 1666-1672
Returning to Cambridge in 1666, and discussing there his new discoveries, Newton wins an immediate reputation. In 1669, when still short of his twenty-seventh birthday, he is elected the Lucasian professor of mathematics. His lectures and researches are mainly at this stage to do with optics. He invents for his purposes a new and more powerful form of telescope using mirrors (the reflecting telescope, which becomes the principle of all the most powerful instruments until the introduction of radio astronomy).
In 1672 he presents a telescope of this kind to the Royal Society and is elected a member. Later in this same year he describes for the Society his experiments with the prism.
In this famous piece of research Newton directs a shaft of sunlight through a prism. He finds that it spreads out and splits into separate colours covering the full range of the spectrum. If he directs these coloured rays through a reverse prism, the light emerging is once again white. However if he isolates any single colour, by sending it to the second prism through a narrow slot, it will emerge as that same colour, unchanged.
It has often previously been observed that light passing through a medium such as a bowl of water can change colour, but it has been assumed that this colour is imparted by the glass or water.
Newton's reversible experiment proves that the phenomenon is an aspect of light itself. Different wavelengths of light have different angles of refraction, with the result that the prism separates them. White light, containing all the wave lengths, can be transformed back and forth. Light of a single wave length and colour can only remain itself.
It follows from this that the perceived colour of different substances derives from the particular wavelengths of light which they reflect to the eye; or, in Newton's words, that 'natural bodies are variously qualified to reflect one sort of light in greater plenty than another'. The sciences of colour and of spectrum analysis begin with this work, which Newton eventually publishes in 1704 as Opticks.
Newton and gravity: 1684-1687
In 1684 Edmund Halley visits Newton in Cambridge. Hearing his ideas on the motion of celestial bodies, he urges him to develop them as a book. The result is the Principia Mathematica (in full Philosophiae Naturalis Principia Mathematica, Mathematical Principles of Natural Philosophy), published in 1687. When lack of funds in the Royal Society seems likely to delay the project, Halley pays the entire cost of printing himself.
The book, one of the most influential in the history of science, derives from the young Newton's speculations about the moon during his time at Woolsthorpe Manor two decades earlier.
The question which stimulated his thoughts was this: what prevents the moon from flying out of its orbit round the earth, just as a ball being whirled on a string will fly away if the string breaks? The ball, in such an event, flies off at a tangent. Newton reasons that the moon can be seen as perpetually falling from such a tangent into its continuing orbit round the earth.
He calculates mathematically by how much, on such an analogy, the moon is falling every second. He then uses these figures to calculate, on the same principle, the probable speed of a body falling in the usual way in our own surroundings. He finds that theory and reality match, in his own words, 'pretty nearly'.
The word gravity is already in use at this time, to mean the quality of heaviness which causes an object to fall. Newton demonstrates its existence now as a universal law: 'Any two particles of matter attract one another with a force directly proportional to the product of their masses and inversely proportional to the square of the distance between them.'
With this observation he introduces the great unifying principle of classical physics, capable of explaining in one mathematical law the motion of the planets, the movement of the tides and the fall of an apple.
The Leyden jar: 1745-1746
The researches of William Gilbert, at the start of the 17th century, lead eventually to simple machines with which enthusiasts can generate an electric charge by means of friction. The current generated will give a stimulating frisson to a lady's hand, or can be discharged as a spark.
In 1745 an amateur scientist, Ewald Georg von Kleist, dean of the cathedral in Kamien, makes an interesting discovery. After partly filling a glass jar with water, and pushing a metal rod through a cork stopper until it reaches the water, he attaches the end of the nail to his friction machine.
After a suitable amount of whirring, the friction machine is disconnected. When Kleist touches the top of the nail he can feel a slight shock, proving that static electricity has remained in the jar. It is the first time that electricity has been stored in this way, for future discharge, in the type of device known as a capacitor.
In 1746 the same principle is discovered by Pieter van Musschenbroek, a physicist in the university of Leyden. As a professional, he makes much use of the new device in laboratory experiments. Though sometimes called a Kleistian jar, it becomes more commonly known as the Leyden jar.
Within a year or two an improvement is made which gives the capacitor its lasting identity. The water in the vessel is replaced by a lining of metal foil, with which the metal rod projecting from the jar is in contact. Another layer of metal foil is wrapped round the outside of the jar. The two foils are charged with equal amounts of electricity, one charge being positive and the other negative.
The principle of plates bearing opposite charges, and separated only by a narrow layer of insulation, remains constant in the development of capacitors - much used in modern technology.
Watson and Franklin: 1745-1752
In 1745 the Royal Society in London awards its highest honour, the Copley medal, to William Watson for his researches into electricity. It is the fashionable subject of the moment, and is about to become more so with the development of the Leyden jar.
In 1747 Watson sets up an ambitious experiment to discover the speed at which electricity travels. He arranges an electrical circuit more than two miles long, linking the positive and negative metal foils of a Leyden jar. There seems to be no measurable difference between the completion of the circuit and the moment when an observer at the middle of the loop feels the shock. Watson concludes that electricity is 'instantaneous'.
His conclusion is not an accurate description of the flow of electricity, but the experiment is nonetheless impressive. As the leading figure in electrical research, Watson is now in touch with an enthusiastic experimenter on the other side of the Atlantic, Benjamin Franklin.
Watson and Franklin independently arrive at a new and correct concept of electricity - that instead of being created by friction between two surfaces, it is something transferred from one to the other, electrically charging both. They see electricity as the flow of a substance which can be neither created nor destroyed. The total quantity of electricity in an insulated system remains constant.
Franklin, a scientist with a popular touch, coins several of the terms which are now standard - positive and negative, conductor, battery (in the sense of a series of Leyden jars linked for simultaneous charge or discharge). His papers on the subject, gathered and published in 1751 as Experiments and Observations on Electricity, become the first (and perhaps only) electrical best-seller. Widely read in successive English editions, and translated into French, German and Italian, this short book makes Franklin an international celebrity.
His reputation is further enhanced, in the following year, when he devises history's most dramatic, and dangerous, electrical experiment.
The new Leyden jars are powerful enough to generate a spark which is both visible and audible. It occurs to many that this effect may be the same as that generated in nature in the form of lightning. Franklin invents a way of testing this idea.
In Philadelphia, in 1752, he adds a metal tip to a kite and flies it on a wet string into a thunder cloud. The bottom of the string is attached to a Leyden jar. The point is made when the Leyden jar is successfully charged. For the popular audience Franklin makes the effect visible. He attracts sparks from a key attached to the line. His fame soars. (But the next two people attempting the experiment are killed.)
In conducting his experiment, Franklin already has in mind a practical application if the science proves correct. He reasons that if celestial electricity can be attracted to a metal point, then a rod projecting from the top of a church steeple, connected by a metal strip to the earth, could serve as a conductor for any stroke of lightning and thus save the building from harm.
When the British army proposes to construct a magazine at Purfleet for the storage of gunpowder, William Watson recommends that this highly explosive building be protected by one of Benjamin Franklin's lightning conductors. The proposal is accepted. The science of electricity finds the first of its myriad eventual roles in everyday life.
Joseph Black and latent heat: 1761
Joseph Black notices that when ice melts it absorbs a certain amount of heat without any rise in temperature. He reasons that the heat must have combined with the particles of ice and still be present in the water at 0°C. Heat of this kind (as Cavendish later perceives) consists of greater activity among the molecules, in a form of energy which will be transferred again if the water freezes.
Black calls this phenomenon latent heat, and teaches it in his lectures at the university of Glasgow from 1761. An important discovery in itself, it also enables him to be the first to distinguish between heat (energy transferred from a warmer to a colder object) and temperature (the amount of energy present at a given moment).
