HAPPY BIRTHDAY ELECTRON

Electricity was being exploited well before the force behind it was discovered. Nonetheless, says Martin Redfern, we all have good reason to be celebrating the centenary of the electron
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When the electron was discovered 100 years ago, it was seen as a discovery for basic physics rather than of practical importance. After all, electricity had been put to good use for many years without needing to know what carried it. A century later, the ramifications of its discovery are immense. It is the basis of electronics and lasers, and every television and computer, as well as modern medical imaging including X-ray machines and Cat scanners.

Today the tube with which J J Thompson discovered the electron sits in a case on a landing of the 70s building that now houses the Cavendish Laboratory of the Physics Department of Cam-bridge University. Blown into a bulb at one end with a smaller bulb at the other, it has protrusions with wires sticking out, which are held in place by sealing wax. In fact, it's a replica, the real one resides in a walk-in cupboard down the corridor.

One hundred years ago the Cavendish Lab-oratory was a very different place. Then in the centre of Cambridge next to Free School Lane, the noise of carriages and passing undergraduates meant that the more delicate experiments had to be performed at night. Before it was founded in 1874, with the donation of pounds 8,450 by the Duke of Devonshire, Chancellor of the University and a member of the Cavendish family, the only experimental physics in Britain was performed in the private laboratories or homes of those who could afford it. The nearest most students came to practical science was mathematics.

Like the two directors of the Cavendish, James Clerk Maxwell and Lord Rayleigh, who preceded him, J J Thompson came from a background of theory and mathematics. He was appointed Cavendish Professor in 1884, when he was 28. "Matters have come to a pretty pass when they elect mere boys as professors," one senior member of the university remarked. J J, as everyone called him, had little experience of experimental physics and appears to have been rather clumsy, getting students to do the more fiddly work.

The phenomenon that Thompson was working on had already been studied for 40 years. It involved creating an electrical discharge through a gas. This was done at low pressure in an evacuated tube. At either end of the tube was an elec- trode, a conductor through which electricity could enter or leave the tube. One electrode, a cathode, was charged with high negative voltage. The cathode appeared to produce discharge into the gas that was attracted towards the electrode at the other end of the tube, the anode (a positively charged electrode). Some of the discharge, called cathode rays, would pass on through a hole in the anode to produce a fluorescent glow on the far end of the tube. These were often known as Crookes tubes and, as early as 1879, William Crookes had suggested that the mysterious cathode ray might be a stream of particles.

Thompson then managed to deflect the ray produced by the tube, both with an electrical field applied to plates further down the tube, and with magnets. As he knew the force of the magnet, he was able, using simple mathematics, to deduce that the rays must be of particles lighter than the smallest atom, yet carrying an electrical charge. He went on to experiment using different gases and different materials for the electrodes, but the results remained the same, proving to Thomp-son's satisfaction that what he had discovered was a fundamental particle found in all forms of matter. He announced his discovery, not through a scientific journal, but by demonstrating it at the Royal Institution in London - perhaps the Victorian equivalent of a press conference.

Today, J J Thompson is almost universally credited for the discovery of the electron, and indeed in 1906, he won the Nobel prize for it. However, Dr Graeham Gooday, science historian at Leeds University, thinks that credit only really came after the First World War. By this time, he suggests, many of Thompson's students from the Caven-dish had spread around the world telling stories of their illustrious director as they went. Gooday suggests that "the stories fitted the ideal of how a scientist should work and thus proved popular, but really represented a form of intellectual colonialism on the part of Thompson's students and a degree of amnesia about the role of physicists in the rest of Europe." But retired physicist Gordon Squires, curator of the collection which includes Thompson's cathode ray tube and admittedly himself a Cavendish physicist, points out that it's no good just doing good experiments if you don't have the vision to interpret them correctly. It was Thompson who believed his own results had indeed found a fundamental particle 2,000 times lighter than the nucleus of the lightest atom.

But Thompson was by no means the only player in the field. Experiments similar to his had been performed in Germany by Emil Wiechert who had also measured the magnetic deflection of cathode rays. Four months before Thompson's announcement, Wiechert proposed the particles were between 2,000 and 4,000 times lighter than hydrogen, the lightest atom. But he did not try changing the gas or the electrodes to see if the particles were fundamental.

Walther Kaufmann in Berlin also made measurements before Thompson's announcement, getting results within 1 per cent of the present value for the charge to mass ratio of the electron, compared to typical errors of 30 per cent in Thompson's values. However, he was convinced that the rays must be waves or vibrations rather than particles because they seemed so light. And at Leiden University Pieter Zeeman found that features in the spectrum of light produced by sodium atoms were smudged or broadened when a magnet was held near the source. Zee-man suggested that the magnet was affecting the motion of small, electrically charged particles. He even estimated the ratio of their charge to mass, announcing in October 1896, a value similar to that found by Thompson the next year.

The name electron was first used in 1891, not by Thompson, but by Irish physicist, George Stoney. However, Stoney, a theorist, did not follow up his suggestion of such particles with experiments. Thompson insisted on calling his particles ecorpuscles and proposed that they must be embedded in atoms in a sphere of positive electricity, like fruit in a plum pudding. It was not until 1911, when his student, Ernest Rutherford, also at the Cavendish, discovered the atomic nucleus, that Thompson was proved wrong.

Rutherford fired alpha particles, now known to be nuclei of helium atoms, at gold foil so thin that it was almost transparent. Most of the particles went straight through, but occasionally, they bounced back almost in the direction from which they had come. Rutherford compared his surprise at this to firing a large naval gun at a sheet of tissue paper and having the shell bounce back. It showed that almost all the mass of an atom is concentrated in a very small nucleus with only the lightweight electrons outside it.

Thompson continued his studies of electrical discharges in cathode ray tubes. Among other things, he experimented with positive rays produced from the ionised gas within the tube. In 1910 he showed that neon contained atoms of two different weights, later called isotopes. His son, George, also a physicist, studied the way in which electrons seem to interfere with one another as if they were waves. It is ironic that J J was awarded the Nobel prize for showing that the electron was a particle, whereas George received nothing for showing that they behaved as waves.

The cathode ray experiments did not measure the charge of the electron or its mass independently, but the ratio between them. Thompson estimated the mass to be two thousandths of that of the hydrogen atom by assuming the charge. But he suggested a way in which the charge might be measured independently and this was done over a seven-year period up to 1913 by Robert Millikan in Chicago.

Millikan produced tiny, electrically charged droplets of oil and managed to suspend them between two metal plates by adjusting the electrical charge between the plates, watching the droplets through a microscope. After thousands of measurements, he found that the charge needed to hold a droplet still was always a precise multiple of the same value: the charge on an individual electron. Looking back at Millikan's notebooks there are some signs that instead of averaging the data from all his measurements, he was selective, rejecting values that seemed different from the one he wanted. In spite of this his value has remained the standard. But that could soon change.

At the Cavendish Laboratory, Professor Michael Pepper and his colleagues are now working on a new and far more accurate technique. They describe it as a quantum version of the Millikan experiment. If high frequency sound waves of a precise frequency are fired into the semi-conductor, gallium arsenide, they can carry electrons with them. It's rather like waves on the ocean carrying surfers on their crests and sweeping them in to the shore. In fact, the electrons do not ride on the crests of the waves, but are trapped in the troughs. The Cavendish scientists have now refined this principle by developing a way of etching a very thin line on to a gallium arsenide chip. The electrons are then squeezed into line upon it. It's rather like reducing the number of surfers on top of each wave until there is only one per wave. Since the frequency (the number of waves per second) and the electrical current produced by the electrons can be measured with extreme accuracy, so can the charge carried by each electron.

So far the researchers haven't quite squeezed all their electron surfers into single file but they have proved that the technology works and that each time they reduce the number of electrons, the current drops by a measurable amount. This has already given them a better value for the charge on the electron than that of Millikan and they hope it will lead to a new world standard on which all electrical units can be based.

This lack of precision in the measurement of the electron has not restricted its applications. The same basic technique that Thompson used in his tubes was later used in radio valves which formed the essential components for radio transmitters, receivers and amplifiers, and also in the tubes found in television sets and computer displays. To this day, they are called cathode ray tubes.

Another device which has at its heart something similar to a cathode ray tube is an electron microscope. Invented in Germany just before the Second World War, the technology was supplied to the Cavendish Laboratory after the war as part of reparations. There its advantage over the light microscope was soon apparent. The shorter wavelength of the electrons made much higher resolution possible in much smaller samples. The electrons could also penetrate materials opaque to light and soon they were being used for detailed analysis of flaws within metals, something vital for the fledgling aerospace industry.

It is even possible to use electrons to see electrons. If, instead of scanning a beam of electrons across a sample, the electrons come from an ultra-fine point held very close, they will jump into the sample by a process called tunnelling. The so-called tunnelling electron microscope can reveal the lumps and bumps of individual atoms on a surface. It can even show the ripples of electrons around them. As such, it has become the ideal tool for watching the smallest electronic components as they operate. It can reveal channels of electrical current through the semi- conductors that make up the most sophisticated electronic chips of today. And beams of electrons can now be used to make those chips too. Where circuit components used to be printed on silicon using photographic techniques that were limited by the wavelength of the light, scientists at Bell Labs in the USA have devised a way of writing the patterns at high speed with a beam of electrons.

For the ultimate application of electrons of the future, we come back yet again to the Cavendish Laboratory. There Professor Haroon Ahmed and colleagues in research sponsored by the Japanese company, Hitachi, are devising a memory device so compact that it could, in principle, store its memory in single electrons. Imagine a computer that produces high quality, 3-D moving pictures of a world you can appear to walk through and interact with as if it were real, or a feature film you can take part in, talking to the characters, flying their space ships and so on. Discs and even CDs are simply too slow to access but with a single electron memory, this could be feasible.

Already, there are memory chips that could store the works of Shakespeare. But predictions suggest that virtual reality computers next century will need so much memory that you'd have to pave a tennis-court with the best chips of today to supply it. Each memory cell is like a switch and the present chips take about half a million electrons to switch one cell on or off. That not only takes up space, but gives off heat. By using what's called a multiple tunnel junction in a chip, not of silicon but of gallium arsenide, electrons can be stored on tiny islands, revealing their presence without them escaping. Already, the scientists have confined between 10 and 100 electrons on each island and believe they can go further still. An entire library of text and pictures on a single chip could yet become a reality.

Now, just as the electron celebrates its birthday as the first fundamental particle to have been found, research in Germany is suggesting that it may not be fundamental after all. HERA, an underground atom smasher near Hamburg, has been colliding electrons and protons at almost the speed of light for the past five years. By present theory, the three quarks that make up a proton and the single lepton, which is what an electron is, should be distinct and fundamental forms of matter. But physicists have been puzzled why three quarks should carry exactly the same but opposite charge to one lepton. Now there is brief and tantalising evidence from HERA of a particle that combines the characteristics of both. If such a lepto-quark can really exist, then it must imply that both quarks and leptons have even more fundamental components in common. Perhaps, next century, there will be the centenary of another fundamental discovery to celebrate.

! Martin Redfern is Executive Producer for the World Service in BBC Science

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