Case studies from the history of physics

Teaching Guidance for 14-16

Science has no one method. Indeed, Einstein is quoted as saying that a practising scientist may appear to trained philosophers “as a type of unscrupulous opportunist”. What all scientific explanations have in common, however, is that they are testable against evidence from the physical world.

Particular scientific discoveries demonstrate the many ways that observations and scientific explanations become established or refuted. Creative insights of individual scientists, already established ideas, and new instruments and techniques all contribute to new discoveries. Peer review can be hostile; the relevant scientific community can be a very critical audience for new observations or ideas.

In several historical cases, the conventional story is that a discovery was a happy accident. Research reveals that it was nothing of the sort, yet these tales are endlessly repeated in print. Usually, it turns out that several people were coming to the same conclusion and there may have been arguments about who should get the credit.

Energy
Energy and Thermal Physics

Galileo's pendulum

Teaching Guidance for 14-16

Galileo was interested in predicting how bodies move. He allowed an object to roll down and up a curved track, and showed that it rose to roughly the height from which it was released, regardless of the shape of the track.

Galileo recognized that, unfortunately, the experiment was marred by the effects of friction.

Experiment: Demonstrate Galileo's rolling ball

Scientists seek to demonstrate phenomena clearly. They try to eliminate any undesired external influences (in this case, friction), in order to show an underlying principle.

Galileo's rolling ball


Why is a ball a good object to use here? Why might a toy car be worse? Or a block of wood?
A ball is a good choice because it is likely to roll with very little friction, provided it does not slip on the track. Toy cars and blocks of wood suffer more from the effects of friction. In the demonstration, the observer would have to accept that they would reach their original height if there were no friction.

Galileo went on to use a pendulum to demonstrate the same phenomenon. He believed that this would be even less affected by friction.

Experiment: Set up a simple pendulum

Demonstrate how it swings.

What effect would friction have on a pendulum like this? How can you tell that friction is having little effect?
Air resistance and friction at the point of attachment might slow down the pendulum. The amplitude of the pendulum changes only slowly
  • the effect is probably not noticeable from one swing to the next.
  • Experiment: Demonstrate Galileo's pin and pendulum

    Galileo's pin and pendulum


    What is the principal observation of this experiment?
    The bob (mass) always returns to its original height, regardless of the original height of release or the presence of the interrupting rod.
    How would scientists explain this today, using the idea of energy?
    The bob has GPE at its highest point; this is transferred to KE as it moves downwards and back to GPE as it rises again. Since energy is conserved and none is lost from the system (in the absence of friction), it can return to its original height.

    In fact, the idea of the conservation of energy was far in the future when Galileo described and explained his experiment. Instead, he talked in terms of momento and impeto. These terms correspond (more or less) to what is now called momentum.

    Later, Newton also used the idea of momentum, which he regarded as the fundamental property of a moving object. The idea of kinetic energy was not established until the mid-19th century, 200 years after Galileo's death.

    Ideas like that of energy, which scientists today take for granted, are not self-evident. Both momentum and kinetic energy were identified when people realized that they were conserved quantities in certain situations.

    Acknowledgement

    We are grateful to David Sang, author of this Case Study.

    Energy Transferred by Radiation
    Quantum and Nuclear

    Henri Becquerel discovers radioactivity

    Physics Narrative for 14-16

    Henri Becquerel discovered radioactivity in 1896. Fortunately, he kept a detailed diary of his experiments. This shows that the frequent claim that his discovery was a chance event misrepresents his systematic approach to experimentation.

    One approach to this topic might be to act out Becquerel’s sequence of experiments, with lumps of (non-radioactive) rocks and ready-made ‘photographic films’ to show his results. His diary entries would act as a script.

    Ask students to explain the design of experiments, and the results, and to suggest the next step.

    Phosphorescence

    Becquerel was interested in phosphorescence. After exposure to visible light (or other radiation), some materials glow. The glowing fades gradually. His father and grandfather had both worked in this field since the 1830s.

    Henri had some crystals of substances containing uranium. They glowed in the dark, without showing signs of fading.

    Question: How do ‘glow-in-the-dark’ stars work?

    Answer: Stick them to a bedroom ceiling; during the day, they are exposed to light. At night, they glow. Their brightness decreases over time.

    Experiment: In a darkened room, show how a cathode ray tube screen (TV or oscilloscope) fades after switching off.

    Question: Sketch a graph to represent this.

    Answer: An exponential decay graph.

    Question: Use ideas about energy to describe what is going on.

    Answer: Radiation (light, electron beams, etc.) transfers energy to the material. The energy is stored, and released more and more slowly. Of course, the energy is being released even during the irradiation; the material doesn’t wait until dark before starting to glow, but you cannot see it in daylight.

    Becquerel’s first experiment – 24 February 1896

    I took two crystals of potassium uranium sulfate and left them in the sunlight for several hours. Then I placed them, with a coin, on top of a photographic plate. My prediction was that the plate would be blackened by the light from the glowing salts. The coin would block the light from part of the plate.

    When I developed the plate, I could see the outline of the crystals and the shadow of the coin. My prediction has been proved correct.

    I was worried that perhaps chemicals from the uranium had been affecting the plate. Accordingly, I repeated the experiment, this time with a thin sheet of paper between the crystals and the plate to prevent chemical contamination. When I developed the plate, I saw the same effect as before.

    One of the plates used by Henri Becquerel in his experiment.

    Note how Becquerel explains the design of his experiment, makes a prediction, and tests it. Then he tests an alternative idea.

    Becquerel’s second experiment – 26-27 February 1896

    I wanted to repeat my experiment with the uranium salts. I set the crystals on fresh photographic plates by the window, but it was a dull day. No sunshine. I put the plate away, and tried again the next day. Still no sunshine.

    On 1 March, I decided to develop these plates, expecting to find the images very feeble. On the contrary, the plates were blackened with great intensity.

    This made me think that the crystals might be able to affect the plates without exposure to sunlight. Perhaps the action could even take place in darkness. This gave me an idea for another experiment.

    In popular accounts of Becquerel’s work, it is often suggested that he unaccountably developed some plates which had been left in a drawer with uranium-containing rocks. There was an element of luck in his discovery, but each step in his work is logical and understandable. By this stage, Becquerel had made his vital observation and was ready to explore it systematically.

    Becquerel’s third experiment – 1 March 1896

    I took several photographic plates and several crystals of uranium salts. I placed one crystal directly on a photographic plate; a second I placed with a sheet of glass between it and the plate; a third I placed on a thin aluminium sheet on top of the plate.

    I then placed all three in the dark for several hours. When I developed the plates, I found these results:

    • The first plate, where the crystal was in direct contact with the plate, showed strong blackening.
    • The second plate showed slightly weaker blackening.
    • The third plate showed blackening which was much weaker, but nevertheless very clear.

    I am now convinced that uranium salts produce invisible radiation, even when they have been kept in the dark. They do not need to be exposed to sunlight to produce this effect.

    X-rays had been discovered in 1895 by Wilhelm Röntgen. They had been exploited very rapidly. Invisible radiations were suddenly of great interest. This greatly encouraged Becquerel to press on with his work.

    Question: What other invisible radiations were known in 1896? Which have been discovered since?

    Answer: Infra-red, ultraviolet, radio waves, X-rays. Later: Alpha, beta, gamma, microwaves.

    Becquerel’s fourth experiment – 9 March 1896

    I have kept uranium crystals in the dark for 160 hours. There is no sign of any decrease in the intensity of the radiation which they produce.

    Further experiments

    Becquerel went on to investigate a number of uranium compounds, as crystals, liquids and solutions. All produced invisible radiation, suggesting that it was the presence of uranium which was necessary to produce the effects observed. Metallic uranium had the greatest effect.

    He also showed that the radiation removed the charge on a body charged with static electricity.

    The idea that an element such as uranium can produce radiation continuously, without any obvious source, is a hard one to accept – it appears to break the principle of conservation of energy (see guidance note on this). (Phosphorescence, on the other hand, clearly does not, as exposure to sunlight is needed to ‘charge up’ a phosphorescent material.) However, Becquerel’s experiments convinced him that he had indeed observed a radically-different phenomenon.

    Question: Compare the phenomena of phosphorescence and radioactivity. What are their similarities and differences?

    Answer:

    Similarities: Release of radiation, decay over time (although Becquerel couldn’t observe this with uranium).

    Differences: Phosphorescent materials produce visible radiation and need 'charging up'; radioactive materials produce invisible radiation and do it spontaneously.

    Other experiments

    Around the same time, in the USA, Sylvanus P Thompson noticed that uranium salts blackened photographic plates. In Scotland, Lord Kelvin reported that uranium could discharge static electricity. However, neither carried out a systematic study like that of Becquerel, which is why he is credited with the discovery of radioactivity.

    On the evening of 1st March 1896, Becquerel gave his first report to a meeting of colleagues in Paris. At the same time, Kelvin was reporting his findings to a meeting in Edinburgh. Scientists rarely work in isolation on a topic which only they are interested in. There is often a tension between reporting in public on what you have discovered (which may help your competitors), and keeping your findings secret (which may mean that they get their report out first, and can then claim the credit).

    Acknowledgement

    This case study is based on a unit of work in Henri Becquerel and the Discovery of Radioactivity, by D Sang, J Sutcliffe and M Whitehouse, Association for Science Education, 1997, ISBN 086357 2707.

    Extracts from Becquerel’s diary can be found in The Faber Book of Science, ed. John Carey, Faber and Faber, 1995, ISBN 0571 16352 1.

    Electromagnetism
    Electricity and Magnetism

    Oersted, electric current and magnetism

    Teaching Guidance for 14-16

    In 1820, Hans Christian Oersted performed an important experiment which showed that there was a connection between electricity and magnetism. When a current was switched on through a wire, it made a compass needle turn so that it was at right angles to the wire. The current had produced a magnetic field strong enough to cause the compass needle to turn.

    Background to the experiment
    It was already known that an electric current in a wire has a heating effect, and may cause the wire to glow. This showed that the three phenomena of electricity, heating and lighting were connected.

    Scientists seek to ‘unify’ apparently different phenomena by finding their underlying causes.

    Question: How can we show that an electric current has a heating effect, and can also result in light?

    Answer: See experiment

    Heating effect of a current.


    It was also known that when lightning, a form of electricity, struck a ship, the ship’s compass might be affected – its polarity might be reversed.

    Question: What does this suggest?

    Answer: There is a link between electricity and magnetism.

    Scientists draw on evidence from everyday life; they try to understand the evidence through experimentation.

    How could this link be demonstrated in the lab?

    Experimenters imagined that, if an electric current flowed along a straight wire, then the magnetic field it produced would be in the same direction. So they placed their compass needles at right angles to the wire, thinking they would be deflected by the current so that they became parallel to the wire. They saw no effect because, as Oersted was to show, the magnetic field produced is at right angles to the wire. So preconceptions prevented earlier experimenters from seeing the effect. (Note that the available equipment –voltaic piles, typically – would only produce a small current, so the effect would in any case have been very weak.)

    Experiment: Set a compass pointing north-south; place a wire above it, lying east-west. Connect to a 1.5 V cell. There should be no effect.

    Preconceptions (existing ideas) may make it difficult to make progress.

    Oersted’s thinking
    Oersted imagined an electric current ‘struggling’ through a wire. As it flowed, this ‘conflict’ gave rise to 'heat' (or infra-red radiation, as we now understand it) and light, which radiated away from the wire. Might it not also result in a magnetic field, radiating away?

    One idea leads to another, which can then be tested.

    Question: How would you draw magnetic field lines to represent Oersted’s idea?

    Answer: Radiating out from the wire.

    Question: How should a compass needle behave, if this idea is correct?

    Answer: It might be expected to point radially to the wire. Try it, and the magnet does not do this. So magnetic field lines do not radiate out from a current-carrying wire.

    Oersted’s experiment
    Oersted was giving a public demonstration of the connection between electricity, 'heat' and light. He connected a voltaic pile to a platinum wire and showed that it became hot and glowed.

    As he lectured, his idea about a connection between electricity and magnetism resurfaced in his mind. He had a compass to hand (for other experiments he intended to demonstrate), so he decided to test his idea there and then.

    Experiment: Set up the previous experiment, but with the wire lying north-south above the compass, parallel to the compass needle. Switch on – the needle rotates to lie east-west.

    See also:

    Oersted's experiment.


    Oersted reversed the current; the needle moved the other way.

    The effect observed by Oersted and his audience was small; no-one was very impressed. Oersted was aware that other scientists had been distracted by similar elusive phenomena, and it was three months before he spent any more time on his discovery.

    Scientists are often working at the limits of sensitivity of their instruments. A technique may have to be refined greatly before a phenomenon can be observed reliably.

    What Oersted did next
    Oersted showed that the magnetic field around a current-carrying wire was circular; i.e. the lines of force are circles, centred on the wire.

    Experiment:

    Magnetic field due to an electric current in a wire.


    He went on to show that a thicker wire produced a greater effect. He also showed that materials placed between wire and compass had no effect.

    Question: Why?

    Answer: Thicker wire has less resistance, so greater current. Non-magnetic materials have no effect on a magnetic field.

    In July 1820, he sent a 4-page paper outlining his results, in Latin, to several scientific journals.

    Scientists are expected to share their findings as soon as possible, by publication in journals. In 1820, Latin was a common (shared) language which allowed scientists of different nationalities to understand each other’s work.

    Ampere read Oersted’s report, and in the space of a week had repeated the observations and developed a mathematical theory describing how the magnetic field depends on the strength of the current and the distance from the wire.

    In principle, scientific results are checked by other scientists who repeat the experiments, to see if they get the same results.

    Oersted had observed and described an experimental phenomenon. Ampere took it further by writing a mathematical equation to account for it.

    Question: Oersted had shown that an electric current has a magnetic field around it. What practical applications were developed from this idea?

    Answer: Electromagnets, motors, dynamos, transformers, etc.

    More about Oersted
    Oersted founded the Danish Society for the Dissemination of Natural Science, a society aimed at presenting scientific ideas to the general public. Oersted also played a key role in the founding of the Technical University of Denmark, with the intention of improving the scientific basis of engineering.

    Many scientists seek to share their work with a wider audience. They may also seek to share the technological benefits of their discoveries. He described 1820, the year of his great discovery, as the happiest year of his life.

    Scientists can get great pleasure from their work.

    In 1802, an Italian lawyer called Gian Domenico Romagnosi published an account of an observation similar to Oersted’s. However, his article appeared in a newspaper and it wasn’t taken up by the scientific community. Romagnosi doesn’t appear to have followed up his preliminary findings.

    There are many examples of ‘prior claims’ in the history of science. In practice, the acknowledgement usually goes to the person who publishes a careful, detailed and repeatable account of their observations, at a time when other scientists are ready for the idea, and in a place it will be read and taken seriously.

    Acknowledgement
    We are grateful to David Sang, author of this Case Study.

    Rutherford Scattering
    Quantum and Nuclear

    Rutherford's alpha scattering experiment

    Physics Narrative for 14-16

    This is an historic experiment which is often presented to students with little context. It is suggested that, in 1910, the ‘plum pudding model’ was suddenly overturned by Rutherford’s experiment. In fact, Rutherford had already formulated the nuclear model of the atom before the experiment was carried out; his model allowed him to carry out a mathematical analysis of the data gathered by Geiger and Marsden.

    Working with alpha radiation

    Alpha and beta radiations were identified by Rutherford in 1899. Rutherford and Royds showed that an alpha particle was a helium-4 nucleus in 1909.

    Rutherford knew that alpha radiation had a range of about 5 cm in air, and its range in denser materials had been measured.

    Experiment: Alpha, beta and gamma radiations can be distinguished by their penetrating powers.

    Identifying the three types of ionising radiation


    It was clear to Rutherford and many others that, since alpha and beta radiations came from atoms, they were on the right scale for use as probes of matter at the atomic scale. He and his colleagues developed practical sources of alpha radiation. When scientists are working at the frontiers, it is rare that they can buy their equipment and supplies ‘off the shelf’; each part of the experimental technique has to be devised, and equipment has to be designed and built.

    Models of the atom

    Rutherford and his team were working at Manchester University. At the Cavendish Laboratory, Cambridge, CTR Wilson was developing the cloud chamber, a device for showing the tracks of radiation. It is likely that Rutherford saw examples of such tracks, perhaps as early as 1908. Mostly, alpha particle tracks show hundreds of tiny deflections; occasionally, major deviations can be seen. Rutherford guessed that the many small deflections were caused by collisions with the very light electrons, while the (very rare) big deflections arose from collisions with something representing a more concentrated part of the atom.

    For a discussion of this, see the guidance note:

    Evidence of the hollow atom


    See the experiment:

    Display of cloud chamber photographs


    It will help if students are familiar with the behaviour of colliding objects of different masses.

    Evidence of back-scattering

    It is sometimes asked why, if alpha radiation was expected to pass right through the gold foil, the alpha-scattering experiment was designed to allow the detector to be moved round through an angle of greater than 90°. Of course, if the plum pudding model was correct, back-scattering would not be expected. However, as we saw above, Rutherford expected to observe back-scattering.

    In fact, his colleagues had already observed the reflection of alpha particles by solid materials. This first came about when Geiger was investigating the penetration of mica (a transparent mineral) by alpha and beta radiation; he wanted to use thin sheets of mica as windows for sources of radiation, and in detectors.

    Geiger and Marsden carried out an experiment in which alpha radiation was directed at different metals, and the reflected radiation detected. In their diagram:

    AB is the source of radiation; it is a glass tube containing radioactive gas.

    RR is the metal target, which reflects the radiation.

    S is the scintillating screen which emits light when struck by radiation.

    P is a barrier to prevent direct irradiation of the screen.

    M is the viewing microscope through which the screen is observed.

    Hence Rutherford knew that back-scattering was likely, and designed his experiment to observe it. His calculations allowed him to predict the pattern of the results, if not the actual fraction of the radiation which would be back-scattered at any particular angle.

    There is a strong interplay between experimentation and theory in physics. Experimental results may help to alter theories, but theories also help to design experiments.

    Rutherford’s surprise

    Rutherford is often quoted, describing his reaction to the alpha particle scattering experiment in words such as these:

    "It was as if you fired a 15-inch shell at a sheet of tissue paper and it came back to hit you."

    Could Rutherford have been so surprised, when back-scattering had already been observed? He was an enthusiastic popularizer of his work; it is likely that, with these words, he was trying to show his audience how they might respond to his discovery.

    Acknowledgement

    We are grateful to David Sang, author of this Case Study.

    Magnetic Field
    Electricity and Magnetism

    The magnetic Earth

    Physics Narrative for 14-16

    Robert Norman and magnetic declination

    Robert Norman was a scientific instrument-maker, working in London in the 1560s-80s. Among other devices, he made compasses which were sold for use on ocean-going ships.

    To make a compass, Norman started with an iron needle. He checked that it balanced perfectly on its support, and then he magnetized it using a lodestone. Each time he did this (and he did it hundreds of times), he noticed that the magnetized needle pointed correctly to the north, but that its north-seeking pole also tilted downwards slightly. He added a small weight to the other end, to counter-balance this annoying feature.

    His explanation of the tilt of the needle was that he was somehow lacking in skill, or that the material he was using was imperfect.

    He tried an alternative to the counterweight: he cut a small amount off the other end of the needle. However, it still tilted downwards. What was going on?

    Norman set about investigating the effect properly. He suspended a needle so that it could rotate in a vertical plane. Then he magnetized it, and set it up pointing north-south. It pointed towards the north, but it also tilted downwards! This convinced Norman that the effect was real.

    Experiment: Show a magnet, hung by threads. It turns to point north, but there is no obvious sign of tilt. The support prevents it from tilting. Now show a Magnaprobe, which is a small bar magnet mounted in a gimbal so that it is free to rotate in three dimensions. The magnet is clearly tilted, showing that the Earth’s field has a component downwards into the Earth. (Norman called the tilt inclination; now it is known as declination.)

    Natural phenomena are often far from obvious. Intelligent people can work for a long time before they realize an important feature of their area of work.

    Question: Look at a diagram of the magnetic field of the Earth. How would the tilt of a compass needle vary in the opposite hemisphere? And at the equator?

    Answer: The opposite way in the opposite hemisphere (lines of force out of the ground in the S hemisphere); horizontal at the equator. Robert Norman might have got further with his ideas if he had asked sailors to repeat his work in different places.

    Norman tried another experiment. He mounted his needle on a cork, and added weights so that the whole thing had the same density as water (it had neutral buoyancy). He then submerged it in water, thus cancelling out the effect of gravity. Unmagnetized, the needle showed no effect; when magnetized, it pointed north and tilted downwards. This amazed Norman. He had expected that the needle would be pulled downwards to the bottom of the water by whatever force caused the tilt. He declared that he could find no explanation for the tilt of the magnetized needle.

    William Gilbert and the magnetic Earth

    By 1600, scientists and navigators were aware of the deviation (variation) of the Earth’s magnetic field from true north, and of its declination to the horizontal. It was time to dismiss the ancient notion that a compass needle was attracted to the Pole Star.

    Question: What is the Pole Star, and why is it important?

    Answer: It is a star which appears to be directly above the Earth’s N pole, so that the Earth’s axis of rotation passes through it. Thus the Pole Star always appears in the same place in the sky, while the other stars appear to rotate about it. Useful for navigation (if you are in the northern hemisphere).

    How attractive, that ancient idea! The needle of a compass allowed you to navigate, whether or not you could see the Pole Star, because its needle was attracted to the star. Hence the behaviour of a compass was explained in terms of older ideas, from the science of navigation. Scientific ideas often draw on older ideas; this may give the right answer, or it may lead you up a blind alley.

    Question: Why might we reject this idea (that it is the Pole Star which attracts the compass) today, in the light of Norman’s findings, and other ideas?

    Answer: A compass needle points downwards, into the Earth, not up into the sky; and it points to one side of true (geographical) north.

    William Gilbert came to the conclusion that the Earth itself was magnetic. He made a model of the Earth, a terrella, from lodestone, and showed how a compass would behave at different points around it.

    Gilbert made his terrella to promote his ideas through demonstrations. It also helped him to develop his ideas about the Earth’s magnetism. Many versions of the terrella were made and sold to the public; scientific experiments were becoming a popular pastime. Scientific instrument-makers needed such a market to keep their businesses going.

    Experiment: Fix a bar magnet inside a ball, to represent the Earth. Move a Magnaprobe about it; look for the magnetic poles, where the field is at right angles to the surface.

    Question: Gilbert made a prediction: he said that compasses would be of little use for finding directions close to the Earth’s poles. Why is this?

    Answer: A compass needle is vertical at the poles, so you can’t tell which way is north or south.

    Equipment note

    A Magnaprobe has many uses in a study of magnetism. It can show the three-dimensional shape of the field around permanent magnets, and around electromagnet coils. Note that the Magnaprobe Mark I is more sensitive than the Mark II, and so is more suitable for investigating weak fields like the Earth’s.

    Acknowledgement

    We are grateful to David Sang, author of this Case Study.

    Electromagnetic Radiation
    Light Sound and Waves

    William Herschel and the discovery of infra-red radiation

    Physics Narrative for 14-16

    In 1800, William Herschel published a series of papers describing experiments which led him to identify infra-red radiation, a form of radiation beyond the red end of the spectrum of visible light.

    Background to the experiment

    When visible light falls on a surface, some of it may be absorbed; the surface is warmed. A thermometer left lying in sunlight shows an increasing temperature. Today, we would say that light transfers energy from the Sun to the Earth, and when it is absorbed, energy is stored thermally in the absorbing material.

    We also receive infra-red radiation and ultraviolet radiation from the Sun. However, the nature of these was unknown in 1800.

    William Herschel and his sister Caroline Herschel were astronomers working in Bath. William had been using telescopes to observe the Sun. Of course, this is hazardous, so he used dark glass filters to reduce the intensity of the Sun’s rays. He noticed that even with the filters he could sense a warming effect. He also noticed that some filters seemed to allow through more of the light, while others transmitted more radiation that warmed things up. He wrote that, when observing the Sun, he used:

    ...various combinations of differently-coloured darkening glasses. What appeared remarkable was that when I used some of them, I felt a sensation of heat, though I had but little light; while others gave me much light, with scarce any sensation of heat.

    Herschel set about comparing how well these different glass filters transmitted what he calle 'heat', or infra-red radiation. He needed to know whether his observations were being affected by the filters he was using.

    Scientific observations are made using instruments. The data collected are only as good as the instruments; scientists need to get to know their instruments, so that they understand their limitations.

    Question: What happens when white light is passed through coloured filters?

    Answer: Light of certain colours is observed. Some colours (bands of frequencies of the visible spectrum) are transmitted, others are absorbed.

    Herschel’s experiment

    Before experimenting with his filters, Herschel needed to know about the heating effect of different colours of light. Did different regions of the spectrum have equal heating effects?

    He set up an experiment in which sunlight was passed through a slit and then through a prism (from a chandelier), forming a spectrum on his table. He arranged three thermometers in such a way that the central one could be placed at different points in the spectrum. The other two were positioned on either side, to act as controls.

    Question: What does ‘control’ mean here?

    Answer: Herschel needed to show that any effect he observed was caused by the light falling on the central thermometer. The control thermometers eliminated any general heating of the apparatus which might have been occurring.

    Herschel’s results

    Herschel made repeat measurements with the thermometer bulb in the violet, green and red regions of the spectrum. In each, he observed a temperature rise, which he recorded after 8 minutes.

    Average rise in red: 6.9°F, in green: 3.2°F , and in violet: 2°F.

    He concluded that the red rays of sunlight have a greater heating effect or that there is more of it.

    As the Sun moved across the sky, the spectrum moved across the table. When the central thermometer was just beyond the red end of the spectrum, Herschel noticed that the temperature was even higher than before. What was going on? He concluded that there were invisible rays, coming from the Sun, and refracted by the prism beyond the red end of the spectrum.

    Experiment: Carry out a simulation of Herschel’s experiment. See the experiment

    Herschel's infra-red experiment


    .

    Question: In the light of Herschel’s discovery, what experiments would you suggest carrying out next?

    Answer: Suggestions might include: seeing how far beyond the red the effect extended; testing beyond the violet, testing light from sources other than the Sun.

    What Herschel did next

    Herschel found that the heating effect of these invisible rays was greatest at a point beyond the red end of the visible spectrum, and gradually diminished to zero beyond this point. He also looked beyond the violet end of the spectrum, but found no measurable heating effect.

    However, he was able to publish three papers reporting his results, dealing with the heating effect, reflection and refraction (‘refrangibility’) of sunlight, and showing that the same effects could be observed with light from terrestrial sources.

    Being a series of papers read at the Royal Society, and published in the Philosophical Transactions.

    Scientific papers, as well as being published, were often first read out at a meeting of a scientific body such as the Royal Society (in London). Philosophical Transactions is still published, and is the second oldest scientific publication in the world. Today, many scientific discoveries are first reported at conferences before being formally published.

    Concluding his second paper, Herschel suggested that 'heat' and light were all part of the same spectrum, parts of which we see with our eyes, while other parts we feel as heat on the skin.

    Question: What do we now call this (much extended) spectrum?

    Answer: The electromagnetic spectrum.

    Herschel might have continued to think of light and 'heat' as separate forms of radiation. However, he chose to regard them as two different forms of the same phenomenon. Hence, he united two apparently different phenomena, light and 'heat'. Producing explanations of physical phenomena that are as simple as possible is one of the most general aims of science. In Herschel’s words:

    … we are not allowed, by the rules of philosophizing, to admit two different causes to explain certain effects, if they may be accounted for by one.

    Herschel also suggested that the different colours of light might have different chemical effects, i.e. that they might have different effects in chemical reactions. He was right; this is made use of in photographs.

    Once scientists have a new theory to work with, they use it to predict new effects; this suggests experiments which will help to test the theory.

    Where this led

    Physicists’ understanding of the electromagnetic spectrum has extended gradually over the two centuries since Herschel’s work. Applications are manifold.

    Herschel was an astronomer; his work led directly to the idea that more could be learnt about the universe by detecting wavelengths other than visible light. Today, we have infra-red astronomy, X-ray astronomy and so on.

    Question: Herschel used thermometers to detect and measure the heating effect of light. What is the difference between ‘detecting’ and ‘measuring’? What other instruments are used to detect electromagnetic radiations?

    Answer: Geiger counters for gamma, thermochromic paint for infra-red, photographic film for UV to X-rays, aerials for radio waves, etc.

    New or improved instruments can allow scientists to learn about previously undetectable or unmeasurable phenomena. For example, photomultipliers in space telescopes can measure X-rays from distant, sources across the universe, otherwise invisible.

    Acknowledgement

    We are grateful to David Sang, author of this Case Study.

    Electrical Conductor
    Electricity and Magnetism

    QTC – the discovery of a novel material

    Physics Narrative for 14-16

    QTC is Quantum Tunnelling Composite, a remarkable new material discovered by David Lussey in 1997. Since then, the material has been carefully characterized (so that its composition and functioning are well understood), and its first applications have emerged.

    What is QTC?

    QTC is a material made from particles of a metal (nickel) embedded in a polymer. Its resistance changes dramatically when it is compressed. Uncompressed, it is an almost perfect electrical insulator. When a force is applied, it conducts as well as a metal.

    A QTC ‘pill’ is a small piece of the material, a few millimetres across and 1 mm thick. The graph shows how its resistance changes as a force is applied to it.

    Questions: Over what range was the force on the pill varied? How many orders of magnitude are there on the resistance scale? By what factor did the resistance change?

    Answer: Force varied from 4 to 21 N approx; 7 orders of magnitude on scale; resistance varied by 6 orders of magnitude, i.e. by 10 6.

    Experiment: Look at some samples of QTC; measure the resistance of a QTC pill as you press on it.

    Question: Name another (environmental) factor which affects electrical resistance.

    Answer: A familiar example is temperature – the resistance of a pure metal increases by about one-third between 0°C and 10°C. This is tiny by comparison with the effect of pressure on QTC. A thermistor has a bigger change than this, over a narrow range of temperatures. (Other factors include magnetic field, light level (for LDR).)

    Question: What might such a material be used for?

    Answer: Pressure switches and sensors have many uses – more about this below.

    How was QTC discovered?

    QTC was discovered by David Lussey, working in Darlington (UK). At the time, he was trying to make a conductive adhesive for use in a security system. Computers would be attached by a wire to an alarm; the glue joining the wire to the computer would be conducting, so that if the wire was detached, the alarm would sound.

    David Lussey describes himself and what he was trying to achieve:

    I’m not a scientist but I am a practical person with a technical background from the military. When I needed a conductive adhesive and found there wasn’t one available, I decided to make one.

    To make a conducting adhesive, David mixed metal powders with adhesives in different combinations. One turned out to be very special. When two metal plates were glued together, they did not conduct – the glue between them acted as an insulator. However, when he tried to pull the plates apart, they started to conduct.

    This was very strange and not what I was looking for. So I put that on one side (in fact I threw it on one side!) and it wasn’t until some little while later that I thought, ‘Well, that was a strange reaction.’ I went back and measured it with a meter and found I got something very unusual.

    It was not obvious at this stage that the material had commercial possibilities; nor did David understand how the material worked to produce this strange behaviour.

    Question: How could David go on to investigate his material scientifically?

    Answer: Lots of variables – dimensions of sample, composition, pressure, time (the material’s resistance changes gradually after pressure applied). Also look at its microscopic structure – see below.

    Question: Is this information needed to make practical use of the material?

    Answer: Knowledge of the factors affecting resistance-pressure characteristics seems essential for practical applications. Details of underlying mechanism are not so vital, but insight into this might well suggest further developments.

    Before the twentieth century, most materials were exploited without an understanding of how their properties related to their structures. Now, an understanding of material structures is regarded as most important if they are to be exploited to the full.

    Understanding QTC

    David Lussey wanted to know how his material functioned. He approached a scientist:

    First of all I needed a scientist to verify what I had found, and I found one in the shape of Prof David Bloor of Durham University. I took the first lump of this material that I had made to him and he did the specific tests that he needed to do and drew me some graphs. Those graphs actually formed part of the first patent that we put in on this.

    As well as characterizing the properties of the material, Prof Bloor was able to explain its behaviour. The material’s properties come from the fact that the nickel particles, as seen under an electron microscope, are not smooth. Rather, their surfaces are covered in microscopic spikes. Pressure forces neighbouring particles closer together and a current of electrons can flow between spikes on neighbouring particles.

    The process of electrons ‘jumping’ across a gap from one conducting material to another is known as quantum tunnelling. Hence the material’s name – Quantum Tunnelling Composite, or QTC. (Quantum tunnelling is also made use of in transistors.)

    Question: Why did David Lussey need a scientist to help in his work?

    Answer: A scientist would know how to establish the material’s characteristics in a standard way; he would also have access to the equipment needed to investigate its structure.

    Exploiting QTC

    David Lussey set up a company called Peratech to produce QTC commercially. Working with designers quickly showed the great range of possible applications of the material, including switches, smart fabrics, tactile sensors and electronic noses.

    Experiment: Make a pressure sensor using QTC and calibrate it.

    David Lussey explains how he sees things developing:

    Within a few years I think you will see QTC in different forms perhaps but in many different uses. We can do so many things with it that some of these things are going to very quickly become commonplace. That’s a great drive in itself. That keeps me going, because what I want to see is everybody using it everywhere we can. It’s just the thrill of getting it to market, and making sure it does what it’s supposed to do, that keeps me going. That’s what drives me!

    Question: What differences might there be between what drives a scientist and what drives a technologist?

    Scientists are interested in discovering phenomena and explaining them; technologists are interested in exploiting phenomena in products which can be sold. Of course, a scientist may also be interested in exploitation and a technologist in explanations.

    NASA is researching the use of QTC in developing tactile sensors for robotic hands as part of their ‘Robonaut’ project.

    Acknowledgement

    This page is based on QTC: A remarkable new material to control electricity published by the

    Gatsby Science Enhancement Programme


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