AGM Overview

A light hearted moment during the AGM.

The Fusion Committee welcomed new additions Richard Humphreys and John Taylor at the 2004 AGM. We also elected Professor Nigel Mason to be the first-ever committee member responsible for liaison between Fusion and the OU Physics & Astronomy Department; and we expressed thanks to retiring committee members Eleanor Cowan and Rob Wilby.

More serious discussion at the AGM.

Following the decision to set up a new Science Revision Weekend to cover both physics and chemistry courses (see last newsletter), the AGM elected Digby Tarvin as Fusion's representative on this exciting venture; Digby also takes on the role of Nexus Rep. And we will have a delegate to OUSA Conference this year - Norrette Moore, who also continues to represent Fusion on the Societies Standing Committee. Norrette agreed to continue in the role of Treasurer, but only for one more year. So, budding money-minders, sharpen your pencils for 2005!

Professor Jian Lu shows Fusion members around the Biological Physics Lab at UMIST.

John Pollard packed away his T-shirts to take over the Membership portfolio; Jim Grozier is looking after Merchandise temporarily in addition to organising Events and playing the part of the Society's Secretary; and Paul Ruffle, who has taken on the post of Chair, also continues to have responsibility for the Newsletter and Web site, which have been combined into one post.

The minutes of the 2004 AGM are available here.

More Biological Physics from Professor Jian Lu.

Full AGM Photo Report. Fusion AGM Weekend photos by Lorna Pain.

QUANTA AND CONTINUUM

The Fusion Regional Contact List

The Fusion Regional Contact List is currently being updated. The list is intended to encourage networking between Fusion members to enable local activities to be organized and to act as source of informal support for members during their OU studies. Any member requiring a copy of the updated list can obtain one by sending a stamped addressed envelope to Fusion (Contact List), 92a Springfield Road, Brighton, East Sussex, BN1 6DE. Members with email addresses will be contacted in the near future to ask if they would like to receive a copy by email.

John Pollard, Membership Officer

Ask-a-Boffin

The Physics of the Rainbow

In our Ask-a-Boffin feature in the last newsletter, Ray Ash asked: "We recently saw an OU programme on the physics of the rainbow, which asserted that the fringes of reversed colour inside the rainbow have no complete explanation. Is this still true, in view of the programme being many years old?"

We contacted John Hardwick of Culham Electromagnetics & Lightning, suggesting that the fringes Ray refers to might be the so-called "supernumeraries" - faint, pale higher-order diffraction peaks which appear below the main rainbow. John replied that the supernumeraries are, in fact, fairly well understood in terms of the Airy theory of the rainbow, but queried whether these are indeed what Ray meant as "I think (the colours) are in the same order as the primary?" Another possible candidate is the secondary rainbow, which appears above the main one - its colours are indeed reversed.

Still confused? Then Read John's excellent article "The Subtlety of Rainbows" in the February edition of Physics World, pages 29-33. (If you don't get Physics World, then you need to join the Institute of Physics - fast!).

Absolute Zero

Fiona Mitchell asks "I was wondering, as there is an absolute zero Kelvin, is there a theoretical maximum Kelvin temperature?"

Paul Ruffle replies "I guess there is not a theoretical maximum temperature in degrees Kelvin (K), as at the moment of the Big Bang the Universe was infinitely dense and hot occupying zero volume, although we have no knowledge of that moment, only that of events that subsequently occurred. On a more prosaic level, typical stellar surface temperatures are of the order of thousands of degrees Kelvin. Our Sun (a yellow dwarf star) is around 5,500K at the surface, although a lot hotter at the core (10,000,000K). The surface temperature of red giants are cooler at 4,000K with the hottest blue stars at over 30,000K."

TRIP REPORT

Station X - Fusion's Intrusion

by Richard Humphreys

On the 21st of February 2004, 20 members of an elite force, known as Fusion, supported by three Imperial College members, were led around Bletchley Park, a secret wartime base, to infiltrate the mathematical secrets of military code breaking. Using the knowledgeable insider they discovered such secrets as why more women worked at the famous site than men (the men were abroad fighting, or was it that they required intelligent people!) of how systems designed at the site were later used by superstores (pneumatic tubes) and how they stopped draughts from ruining the ladies perms. There was also some information about how the Enigma machine worked and the variations of the axis powers machines (three cogs of five as standard, eight cogs for Hitler and his high command, and a single fixed disc added later), the allied attempts to break the Enigma codes, including the first breakthrough by three Polish mathematicians.

There was a toy museum and model boats that kept those young at heart occupied, while the mathematicians spent some time puzzling over the information dotted around the varied and informative displays. Those of us who got lost from such delights in the surprisingly large complex found volunteers re building the colossus machine, the first computational device, a computer museum where everyone found their first game machine working, Spectrum ZX80 for me, and we enjoyed space invaders as it was meant to be played. Despite the bitter wind we had a fascinating journey into a site that remains much of a mystery today, but which demonstrated the genius mathematical minds that were at work saving the lives of soldiers and civilians in World War two.

NB: Station X was a small temporary listening post built within Bletchley Park which was the tenth post in the system, hence the Roman symbol X.

Information on Bletchley Park can be found at www.bletchleypark.org.uk.

Look out for future Fusion events at this exciting location.

Dr Clive Saunders explains the workings of Thunderstorm Electrification during the AGM Weekend.

Fusion Quiz Answers

(OU references are to unit and page number; Fusion Newsletter references are to volume and issue number). See the Fusion Newsletter Winter 2003 for the original questions.

1. The phenomenon that Erwin Schrödinger described as "not one but rather the characteristic trait of quantum mechanics, the one that enforces its entire departure from classical lines of thought" was entanglement. Fusion 3:2.

2. Baron Loránd Eötvös compared the inertial and gravitational masses of bodies, and showed that they are the same to within five parts in a billion. S357 9:34.

3. Moore's Law is the statement that the speed of computers doubles every 18 months. Fusion 2:1.

4. The spectral line predicted by Hendrik van de Hulst was the 21 centimetre line. It is not absorbed by interstellar dust, and so enables us to "see" (using radio telescopes of course) the rest of the Galaxy more clearly. S281 TV6.

5. When a fluid flows in an open channel, there are only two stable solutions of Bernoulli's Equation and therefore only two possible depths of fluid. A hydraulic jump is a transition from one of these stable states to the other. MST322 6:30.

6. The sky is blue because light is scattered by air molecules at a rate proportional to the fourth power of frequency, so the highest frequencies (the blue end of the spectrum) are scattered most. SMT356 16:16.

7.CMS stands for Compact Muon Solenoid, part of the Large Hadron Collider at CERN. It will weigh 12,000 tonnes when completed. Fusion 2:2.

8. It takes a photon ten million years to get from the interior to the surface of the Sun. It is not the same photon, because each photon is absorbed by an electron and then re-emitted. The energy of the "photon" gradually diminishes (luckily for us!) Fusion 2:3.

9. The underground laboratory is the Gran Sasso National Laboratory. The site is shielded from cosmic rays and has low natural radioactivity. It is used for experiments involving rare events (and therefore requiring a low background) such as double beta decay and proton decay. Fusion 2:4.

10. The 1925 experiment which went wrong was that of Davisson & Germer. As a result of damage to their target caused by a vacuum leak, the crystalline structure was changed and it exhibited the first signs of electron diffraction. SM355 1:11.

11. Robert Millikan cut off the tails of his graphs, to make his investigation into the photoelectric effect fit the theory. Fusion 1:4.

12.Charles H. Townes was awarded the Nobel Prize for physics in 1964 for the (microwave) laser. S271 16:15.

13. Harlow Shapley chose astronomy because it was at the top of an alphabetical list of courses. Fusion 1:3.

14. A snooker player should hit the cue ball 0.7 times the diameter of the ball above the table in order to impart pure rolling motion without spin or skidding. Fusion 3:2.

15. The densest planet in the Solar System is Earth. S281 2:6

16. The tidal effect of the Moon adds half an hour to the length of a day every ten million years. The Earth will eventually slow down to the same pace as the Moon's orbit. Fusion 1:4.

17. The Rayleigh Criterion determines whether two points a distance d apart, a distance u from a lens of diameter D, can be resolved using light of wavelength &lamda;. Numerically, d > 1.22&lamda;u/D. ST291 8:40.

18. The theory that Christian Huygens called "absurd" and Gottfried Leibniz condemned as "occult" was Newton's Theory of Gravity. Fusion 3:1.

19. An instrument that uses the Hall Effect measures magnetic flux density. S271 10:23

20. According to classical physics, the kinetic energy of protons in a star like the Sun is insufficient to overcome the electrostatic repulsion between them, so that they could not undergo fusion, and no elements heavier than hydrogen could be produced. Quantum tunnelling increases the probability of fusion sufficiently to make stars work, and therefore to produce the heavier elements that living organisms (and planets) are made of. SM355 6:15.

AGM Weekend in Manchester

Fusion members and guests outside the Manchester Museum of Science and Industry.

A whole weekend of Physics in the company of Fusion members! Who could refuse. So up to Manchester we went. We started the weekend with a presentation and tour of the Godlee Observatory with the friendly and informative members of the Manchester Astronomical Society. An eloquent history of the society and the telescope was given in the wooden dome of the 100 year old structure unceremoniously slotted onto the roof of UMIST, before a fascinating slide show bought the history to life. Whilst we were not lucky enough to have clear skies we revelled in the balcony view overlooking Manchester. With such an interesting start, conversation flowed over the evening meal in the colourful surroundings of Manchester night life.

The sympathetic late start the next morning was opened by Dr Clive Saunders who gave an electrifying talk on his research on Atmospheric Physics, in determining the charge relationships within storm clouds. See his article on Thunderstorm Electrification below. This fascinating subject was then followed by Professor Jian Lu who spoke about his work on Biological Physics, in developing materials and devices for medical procedures. The tour of his laboratories was the icing on the cake. After a well deserved lunch to digest the immense amount of information we bit into the serious business of the Fusion AGM.

Physicists being the tough breed that we are, after this gruelling session we boldly went straight into a lecture about the Chemistry of the Interstellar Medium given by the Head of Astrophysics, Professor Tom Millar. We retired, our heads spinning, to something easier; Simple in the City treated us to a delightful meal which was garnished with an after dinner talk by Dr Ian Saunders of Lancaster University. His anecdotes, which began with his reasons for not giving after dinner talks, kept our spirits up after a long, but happy day.

A fitting conclusion to the AGM Weekend - Sunday lunch at the White Lion pub near the Manchester Museum of Science and Industry!

On Sunday morning we took a sabbatical for an inspirational pilgrimage along the Rochdale canal with historical sermons supplied by the extremely well informed and sponge like memory of Lorna Pain. We were then spoiled by a blue badge guided tour of the Manchester Museum of Science and Industry which we could have spent a week in and still not see everything. (There was even a scale model of the USS Enterprise.) Even the walk back to the hotel was eventful, taking us through the Chinese new year celebrations. All in all the weekend was a fascinating insight into a number of different Physics disciplines where we had the benefit of extremely well informed guides and friendly generous companions. Don't miss next years exciting sequel, flying South for the 2005 new year to Brighton.

Richard Humphreys

Manchester Astronomers and the MAS

Fusion members arriving in Manchester for the 2004 AGM were treated to a tour of the Godlee Observatory, home to the Manchester Astronomical Society (MAS), by Kevin Kilburn. Here, Kevin tells us a little about the Society and some of its more illustrious members.

Any society is only a good as its members. John H. Hindle, who joined in November 1917 and later became vice-president, was, by the 1930s, internationally recognised as one of the leading optical experts of the time. Hindle built many large reflecting telescopes, including a 20.5-inch Newtonian- Cassegrain and a 30-inch Newtonian for his friend, Dr William H. Stevenson, former president of both the BAA and Royal Astronomical Society. The 30-inch was subsequently mounted at Cambridge University Observatory, by arrangement with Sir Arthur Eddington.

Hindle frequently travelled abroad, and always took the opportunity to visit major observatories. While in North America, in 1931, he met with Professor Richey, formerly of the Mount Wilson Observatory and largely responsible for figuring the 100-inch mirror of the Hooker reflector. He also met Dr. George Ellery Hale, in Pasadena, and suggested his new method of testing secondary mirrors for the 200-inch telescope, then under construction. Hindle was invited to see the pouring of one of the 200-inch mirror blanks in 1934. It is tempting to think that had he lived, Hindle would have been invited to Mt. Palomar, in 1948, to see the commissioning of the Hale Telescope. Hindle's reputation as a telescope maker encouraged others in the MAS to make their own instruments, a tradition that is still very much alive.

In 1935, Eric Burgess (b1920 ) joined MAS, and a year later formed the Manchester Interplanetary Society. Eric was Manchester's first 'Rocket Man'. In 1944 Burgess, along with Arthur C Clarke, helped found the present British Interplanetary Society. In October 1945, Arthur C Clarke published his seminal paper describing the potential use of manned orbiting space stations acting as 'extra-terrestrial relays' for radio and TV broadcasting. Eric Burgess followed this the next year in an article published in the November 1946 edition of Aeronautics, proposing to use automatic robotic satellites in geostationary orbits for the same reason and for meteorological and other purposes. It is this technology, first suggested by Burgess, which best describes modern global telecommunications.

After the war, Eric Burgess graduated from what is now UMIST with a combined degree in chemistry and physics. He later emigrated to California where he became a successful part-time science journalist and writer on space exploration. In the 1970s Burgess wrote or co-authored several books on NASA's exploration of the planets with interplanetary probes, a term that was itself coined by him in 1952. In November 1971 he suggested to Carl Sagan that a visual message from all mankind should accompany Pioneer 10. So was born the idea of fixing a plaque to the spacecraft. A similar plaque is attached to Pioneer 11. Eric Burgess last visited Manchester in 1978 when he addressed the Manchester Astronomical Society at its 75th anniversary meeting. He was then in the UK as science adviser in the making of the James Bond movie, Moonraker.

During the latter half of the twentieth century it has to be acknowledged that MAS, like many amateur astronomical societies, can only claim to have made minor contributions to the science in the light of burgeoning professional studies. Nevertheless, individual contributions by its members are recognised. In 1952, Professor Zdenek Kopal was elected to the first chair of Astronomy at Manchester University. He was an honorary member of MAS until his death. Ken Brierley and Morris Marlowe were two of only a handful of observers to witness the June 1954 total eclipse of the sun from the British Isles, from the northernmost tip of Unst in the Shetland Islands.

Other MAS members made significant contributions to astronomy. Wal Sargent was a student at Manchester University in the mid 1950's. He subsequently became director of the Palomar Observatories and Ira S Bowen Professor of Astronomy at Caltech. His contemporary, now Prof. Leon Lucy, devised the Lucy-Richardson deconvolution algorithm that corrected the initial spherical aberration of the Hubble Space Telescope.

In the 1960s and '70s, John Rustige, Ken Bispham and Allan Maudsley were among the first solar observers to make detailed observations of polar faculae with the Godlee telescopes, solar phenomena that still merit research.

In the 1980's, Dr David Whitehouse joined NASA before becoming a BBC science correspondent. MAS president, Dr Peter Mack became assistant professor of astronomy at the University of Oklahoma and manager of MIT instruments at Kitt Peak before setting up a business making research-grade telescopes and instrumentation in Tucson, Arizona.

Last but not least, in the closing years of the 20th century Michael Oates became the most prolific comet discoverer to date, with a tally of over 140 mini-comets discovered by analysing data from the ESA-NASA SOHO satellite. In January 2004 his efforts were rewarded with the naming of a minor planet, (68948) Mikeoates. Mike is also codiscoverer in November 1997, with Tony Cross and the present writer, of Manchester AS's greatest find, a rare star atlas that was to have been published in 1750 as Dr. John Bevis's Uranographia Britannica.

Manchester Astronomical Society is now only one of many dozens of such organisations in the UK, but as it enters its second century, it looks forward to continuing its association with UMIST and internationally in the pursuance of knowledge and the enjoyment of space to the benefit of its members and the public.

Manchester Astronomical Society

Manchester Astronomical Society is one of the oldest provincial amateur societies. Although Leeds AS and Liverpool AS are older, both experienced many years of inactivity. Manchester AS traced its roots directly back to Liverpool AS and its immediate successor, the British Astronomical Association (BAA) formed in 1890. By then Liverpool AS was moribund. In January 1892, the North Western Branch of the BAA was formed in Manchester to accommodate those members living within the Liverpool-Manchester-Leeds axis and taking in Lancashire, north Cheshire and parts of Yorkshire. Many of the Branch members lived in the then affluent Manchester suburbs a mile or so south of the city centre. Some were ex LAS members or members of the Manchester Literary & Philosophical Society.

Fr. Walter Sidgreaves, SJ, FRAS became the first president. Alfred Brothers FRAS, Samuel Okell and Thomas Thorp, FRAS were elected vice-presidents. Within the first two years the Branch was to all intents and purposes flourishing, yet all was not well financially and things went from bad to worse. By the turn of the century it was apparent that the Branch could only survive if it became independent from the BAA. The seeds of the separation were set on 5 November 1902 when members of the Branch met in the observatory atop the Manchester Technical College to examine the new telescopes.

On 18 September 1903, a group of members of the North Western Branch of the BAA gathered in the lower room of the Godlee Observatory and formed the Manchester Astronomical Society. Professor Thomas H Core took the chair and was elected its first president. William C Jenkins, the director of the Godlee Observatory and employed by the college, became the Honorary Secretary.

At Christmas 1919, a difference of opinion between the vice president, William Porthouse and the Principal, J.C. Maxwell Garnett, resulted in MAS vacating the Technical College in favour of 36 George Street, the home of the Manchester Lit&Phil and the former home of John Dalton, the Manchester scientist and chemist. Later, shortly before WWII, MAS moved to the central library but its rooms were soon taken for war work. Following the death of Jenkins, the observatory was visited infrequently and was taken over by fire wardens during the war. It was to be twentysix years before MAS again called the Godlee observatory its home, during which time the Manchester blitz not only destroyed 36 George Street, but forced MAS to suspend its activities, albeit temporarily, during 1941.

The Godlee Observatory

The Godlee Observatory and its twin equatorial telescopes, an 8-inch refractor counterbalanced by a 12-inch Newtonian reflector on the same German equatorial mounting, by Grubb of Dublin, was a gift to the city of Francis Godlee, a local mill-owner and philanthropist. It cost him £10,000. The twin equatorial is the last of only four such instruments constructed by Grubb and designed to maximise the use of a small observatory. The refractor was essentially a visual instrument for lunar and planetary work but was also equipped with a bifilar micrometer to measure the separation and position angle of double stars, allowing computation of binary star orbits and hence relative stellar masses. It later had a spectroscope, a gift from Thorp that enabled star spectra to be examined and surface temperatures to be deduced. The reflector was equipped for prime-focus photography of nebulae. A wide-field plate camera for the photography of comets also augmented the telescopes. Thus the Godlee observatory offered a well-balanced suite of small instruments for the teaching of basic astrophysics and astronomy.

Thunderstorm Electrification

by Clive Saunders, Physics Department, UMIST, Manchester

Thunderstorms have been of interest and concern to man for centuries; surprisingly there is still no generally accepted theory of thunderstorm electrification. Because of the uncertainties, thunderstorm electrification research has continued unabated. In recent years there have been important developments in our understanding due both to studies which have linked the dynamics of storms with the cloud microphysics and to more realistic laboratory simulations of thunderstorm conditions.

Electric Field Changes Produced by Lightning

A major advance on Franklin's discovery that negative electric charge was present in thunderstorms was made in a long series of investigations by C. T. R. Wilson at Cambridge in the 1920's. (Wilson invented the cloud chamber in order to study clouds, but his chamber has proved even more useful for the detection of sub-atomic particles.) He set up a device to measure the Earth's electric field and to record changes in the field when lightning occurred. The theory at the time was that rain is charged positively and so when it falls out of clouds it leaves the cloud negatively charged. However, Wilson showed that "thunderclouds are essentially bipolar, the positive charge tending to be above the negative" so that beneath the cloud the effect of the negative charge in the cloud is to cause an electric field at the ground which releases positive ions by point discharge. These ions are then attracted up to the cloud where they are captured by the falling rain. So Wilson argued that the thunderstorm field causes the positive rain rather than vice-versa.

The identification of the vertical dipole was of great importance, and the general principles of Wilson's method are in use today in multi-station networks of electric field detectors in New Mexico and Florida. When a lightning stroke occurs, electric charge is transferred either between the lower negative charge centre and ground (a ground-stroke) or between the charge centres within the cloud (a cloud-stroke). At the ground, an electric field detector (a flat metal plate connected to an electrometer which allows the induced voltage on the plate to be measured) shows a change in induced voltage when lightning occurs. With several detectors over a wide area, the field change values at each station can be used to determine the location and magnitude of the charge centre which gave rise to the stroke within the thunderstorm.

Such a system has been used in New Mexico to locate the charge centres in storms which were also being scanned by 3 cm radar in order to determine the precipitation structure. They found that negative charge in the range 1 to 20 Coulombs was brought to ground by each ground-stroke (several strokes may follow each other rapidly to form a flash) and that the charges were located between the temperature levels of -9 and -17°C. The charge centres were co-located with regions of high radar reflectivity indicating the presence of solid precipitation particles. Further studies of cloud-strokes in Florida revealed that as the storm developed, the lower negative charge centre remained between the -10 and -15°C temperature levels while the positive charge region was carried aloft in the updraught at 8 m s^-1 through the -40°C level. These results are consistent with the idea that a precipitation mechanism involving ice particles is responsible for the electrification of thunderstorms.

Thunderstorm Studies

A major advance in cloud physics research has been the development of airborne instrumentation to improve our understanding of cloud microphysics. In particular, probes are available from which accurate values of cloud particle concentrations and types may be determined with high spatial resolution. Also, devices to measure the magnitude and sign of electric charges carried by cloud particles have been mounted on research aircraft. So, the electrical and physical properties of thunderclouds can now be related. For example, integrated charge densities have been recorded on precipitation particles in supercooled regions of order one Coulomb per cubic kilometre. This is a substantial fraction of the average space-charge density in those regions where lightning is initiated.

During extensive investigations of thunderstorms in Montana by an international team of scientists, several aircraft penetrated a storm while it was simultaneously probed by ground-based radars. Some of the conclusions are that the electric field developed slowly in the initial stages of the storm but that the first cloud-stroke occurred only eight minutes after field intensification began. They noted that negative charge accumulated at 7 km (-20°C) in regions of high radar reflectivity and that negatively charged particles were carried down in the precipitation. The updraught/downdraught transition zone between -10 and -20°C appears to be a region of charge generation where ice particle collisions occur; the smaller positively charged particles are carried in the updraught while the larger negatively charged particles grow by collecting supercooled water droplets (riming) and reside in the fringes of the updraught near -15°C. Thus the classic Wilson thunderstorm dipole develops with a positive charge region above a negative region located near the -15°C level.

Charging by Ice Crystals

A promising mechanism of thunderstorm electrification, which is consistent with the field observations, involves charge transfer when ice crystals bounce off small hail pellets in the presence of supercooled water droplets. This concept is backed up by extensive laboratory work in which crystals and droplets have been interacted with ice pellets in carefully controlled experiments. The classic work was carried out in New Mexico in the 50's where ice-coated spheres were whirled through a cloud of supercooled water droplets and ice crystals in a laboratory freezer at -27°C. They found that the ice spheres, representing hailstones, charged negatively while positive charge was carried away on the ice crystals when they rebounded.

Figure 1. The cloud chamber and target rods mounted on a rotating frame.

Work in the laboratory in UMIST continues in order to identify the conditions under which hail can charge positively or negatively and to try to identify the charge transfer mechanism. The essentials of the apparatus used in these studies are shown in Figure 1. Gold-plated target rods are moved through a cloud in the chamber which is inside a cold-room. The rods simulate a hailstone and collect super-cooled water droplets which freeze on impact to form a layer of rime-ice. Ice crystals are initiated in the chamber by introducing a metal wire cooled to liquid nitrogen temperature. The crystals grow from the vapour and when they bounce off the target any charge separated is measured as a net current to the target. We learned that the sign and magnitude of the charge transfer depend upon a wide range of variables including temperature, liquid water content, ice-crystal size and the impact velocity. We found a charge-sign reversal temperature which was dependent on the liquid-water content; below this temperature the hailstone charges negatively.

Figure 2. Charge transfer between ice crystals and hailstones leads to regions of opposite charge when the particles move apart in the cloud updraught.

The overall effect of many individual charge transfer events can be incorporated in theoretical models of thunderstorm electric field development. A simple model has shown that the charge separation rate provided by the particle interaction mechanism using the charge transfer values from the laboratory work is adequate to account for the observed values of electric field growth. The resulting thunderstorm charge centre development is sketched in Figure 2. At temperatures below the reversal temperature, hailstones charge negatively; they fall to form the negative charge centre which is usually located between the -10 and -20°C temperature levels. The positive crystals are carried up and form the upper positive charge centre. At temperatures above the reversal temperature, ice crystals charge negatively and are carried in the updraught to reinforce the negative charge centre. The hailstones fall to form a lower pocket of positive charge which has often been observed above the freezing level in thunderstorms and which may help to initiate lightning.

Recent Work

The mechanism by which electric charge is driven across the interface between two colliding particles is still not understood. Our laboratory work shows that the presence of liquid water in the cloud is important in controlling the sign and magnitude of the charge transfer - the accretion of supercooled water leads to heating of the hailstone when latent heat is released. Consequently the diffusional growth rates of the cold ice crystals and warmer hailstone surface are not the same; this provides a physical difference that may be associated with the charge transfer process during the brief collision and separation of the crystals and hail. We have called this the Relative Growth Rate Theory and it seems to be attracting recognition as a likely charge transfer process involving growth rate control of the charge densities on the ice surfaces.

Meanwhile, the ground-rules appear to have shifted. All our concepts of the nature of thunderstorms have come from studies of summer storms, which predominantly bring negative charge to ground. These storms have been the easier ones to study because they occur more frequently, often in known locations. Recently, a network of lightning detectors has been set up across the United States and, of course, it continues to work during the winter months. The results show that a large percentage of the strokes in the winter bring positive charge to ground. There is now debate as to whether the thunderstorm charge centres in these storms are upside down or whether the cloud is sufficiently sheared that the upper charge regions overhang and so discharge directly to ground with positive lightning.

Conclusion

Laboratory work and field studies indicate that the thunderstorm charging mechanism is temperature-dependent and requires the presence of supercooled water droplets, ice crystals, and hailstones. The interactions of these ingredients leads to electrification and ultimately to lightning. The mechanism responsible for the charge transfer is still a topic of considerable debate.

Unnatural Power - The Manchester Museum of Science and Industry

by Bryan Hargreaves

The Setting

The museum is situated in the Castlefield area of Manchester's city centre, so-called because there used to be a castle there. Not a castle but a fort, a Roman fort. For it is on a Bunter sandstone bluff that they established a camp circa 73 AD, the remains of which were clearly visible until the 1760s when the Duke of Bridgewater's canal for carrying coal from his mines in Worsley, begun in 1760, arrived at its final destination on the southern edge of the camp in 1765. The destruction and almost total obliteration of the site followed with the development of the Industrial Revolution, especially that of the railways. The museum, established here in 1983, is the new baby in the cradle of the Industrial Revolution. Incidentally Castlefield is the first area to receive a grant from EU funds as an urban park.

The Site

Essentially it is the site of the Manchester terminus of the world's first dedicated passenger railway service, officially opened by the Duke of Wellington, the Prime Minister of the day, on 15 September 1830. As the original Liverpool end of the track disappeared it is the oldest surviving railway passenger station. Across the lines stands the very first purpose built railway warehouse, a four storey building of five bays designed it is thought by Thomas Haigh of Liverpool. It seems to have been his plan for the Gloucester docks. Thus there is direct connection from canal to railway in the provision of warehousing while the station itself is an adaptation of a typical Georgian town house, next door to the Georgian town house of 1809 bought in 1828 for the Station Agent, the general manager.

Across from the 1830 warehouse is the railway approach, above ground level, to the 1880s Lower Byrom Street warehouse, the official entrance to the museum. After the opening of the Lancashire and Yorkshire railway in 1844 Liverpool Road station ceased to act as a passenger station, replaced by Victoria Station. The warehouse itself is typical of the late Victorian fireproof building of cast iron and brick with jack arches.

To the south of this is the single shed 1855 transit warehouse built for quick expedition of perishable goods, for a large industrial city has a voracious appetite. Without the coming of the railways the growth of Manchester and its immediate area might have been limited through the lack of transport facilities to feed the workers.

Finally across the road facing this building, now named the Power Hall, is the cast iron and glass market hall of 1877 designed by local architects, Magnall and Lockwood.

The Contents: A Taster

The museum offers many different objects associated with the Industrial Revolution and its development associated with Lancashire. Its two main themes are the development of unnatural energy and communications. The Power Hall is the most obvious attraction. Here are locally produced machines which have been recovered and installed - from a Newcomen engine of 1712 invention, via gas machines, and one hot air machine, to electrical generators. There are examples of the steam engines that drove the cotton mills which made Great Britain the most productive and richest country in the world, Manchester and its hinterland being responsible for 50% of the gross national product by 1830. A sample of these engines operates daily: the gas and electric engines in the morning; the steam engines from noon.

In the same hall one of the oldest Rolls Royce cars in the world, built in 1905, is on display. On 4 May 1904 Mr. Henry Royce was introduced by a mutual friend, Henry Edmonds, to the Hon. Charles Royce at the Midland Hotel in Manchester. Over lunch the most famous car firm to date was born. This is the road transport section, connected with the area but overshadowed by the railway locomotives. Alas no Rocket but there is the rebuilt Planet series developed by Robert Stephenson for the Liverpool Manchester Railway, the first to have brakes! The monster Garret from the local Gorton works dominates the scene. There is "Ariadne", a Manchester to Sheffield DC loco, later sold to the Dutch railways. At the exit is a replica of the Novelty which was the crowd's favourite to win the 1829 Rainhill trials, but had a bursting boiler and jumped the rails. In the summer months at the weekends one of the museum's old engines gets up steam and provides a ride round the goods yard for a small fee.

The 1880s warehouse contains history of textile development in Manchester, with examples of machinery from the domestic days, from Kay's Flying Shuttle to the factory machines, Arkwright's Water Frame to the steam power driven machines which replaced them. These cut down machines in the well of the goods warehouse are operated in the afternoon, showing the operation carried out from the arrival of the bails of cotton to the woven material.

The passenger station building itself is largely given over to a history of the development of Manchester from sewers (though without rats) upwards including examples of scientific instruments which were developed at the same time. For the Industrial Revolution fuelled the great burgeoning out of knowledge: Dalton developing his atomic theory in Manchester, founding Chemistry, while his pupil Joule carried out pioneering work which led to the establishment of Physics. There is too a small section on the history of gas with real gas jets from the early 19th century burning. Did you know that the first textile mill to be lit by gas was Lee's in Salford in 1805?

Bryan Hargreaves taking Fusion members and guests around The Manchester Museum of Science and Industry.

To complement the history of gas there is also the Electricity Gallery in the 1830s warehouse which includes the production sources salvaged from defunct power stations to representative rooms in houses both pre- and post-war. But the object not to be missed is a replica of 1948 Baby, remarkably born of two men, Professor F.C. Williams and Tom Kilburn. It is the machine that heralded in the modern electronic Computer Age. Between noon and 2 p.m. on Tuesdays volunteers from the Computer Society operate it. The rebuild was carried out in 1998 at Manchester University by Chris Burton with advice from Tom Kilburn himself, so it's authentic.

The former market hall provides the most modern transport section, the Air & Space museum. Quite fortuitously it was built in 1877 the year of the birth in nearby Patricroft of A.V. Roe, the first Briton to build and fly his own powered machine (Japanese engine) in 1909. An apprentice-built replica of 1953 greets you on entry complete with the sponsor's logo, Bulls Eye, the (elastic) braces firm manufactured by his brother and partner. The triplane looks as if it benefited from them! By its side however is a 1954 English Electric PIA Experimental Jet built, at Preston, the first British jet to break the sound barrier. Compared with the developments in other parts of the museum the two planes show how swiftly in the twentieth century developments nave come, less than 50 years. Two World Wars concentrated minds and investment which led to air travel for the common man as well as massive destruction. On show is a 1930s AVRO 504K based on the 504 of 1913, the most successful First World War plane, 8,000 constructed. It's the updated RAF Lancaster bomber (7,374 built) of Roy Chadwick, Avro's chief designer from 1918, which dominates the hall: the Shackleton AEW 2 (Airborne Early Warning System), the RAF's longest serving plane, overshadowing the most famous World War 2 plane, the Brylcream pilots' Spitfire. This one is the sophisticated 1945 Supermarine version.

Pear-Shaped Neutrons and the Meaning of Life

by Jim Grozier

One of the big questions physicists are asking themselves today is, "why are we here?" Yes, of course, philosophers have been asking that question for centuries; but the modern version of it has a slightly different slant. What is puzzling physicists in particular is not why the Universe is here, but why there is any matter in it. Assuming that the Big Bang created equal amounts of matter and antimatter, these should have simply annihilated each other by now, leaving only energy. One might argue that the matter and antimatter were flung apart so violently that they occupy different parts of the modern Universe - that they simply haven't had a chance to meet up since they were created. But no traces of antimatter can be found, however far we look; and astronomers know the signature of the radiation that would be produced by matter-antimatter annihilation well enough to infer that the antimatter just isn't there. We are forced to conclude that either there was more matter than antimatter in the first place, or that the two behave slightly differently - that there is not a perfect symmetry between them - and that after all the antimatter was annihilated by matter, enough matter was left over to form the galaxies, stars, planets - and us.

So it seems that to explain this mystery we must look into the properties of particles and antiparticles to see if there is any asymmetry between them. The CPT Theorem - a very deeply-held view in particle physics - asserts that the combined operations of Charge conjugation (replacing particles with antiparticles and vice-versa), Parity (replacing left-handed particles with right-handed ones and vice-versa), and Time reversal leave the universe unchanged. This effectively means that an asymmetry between particles and antiparticles will imply a corresponding asymmetry in their behaviour under time reversal. Yes, I know, we cannot perform experiments with time running backwards; but we can imagine it. So, here's a particle physics experiment that you can do at home, on your kitchen table. Take a tennis ball, and spin it. Now take another tennis ball and spin it the other way. That is what you would see if you looked at the first ball with time running backwards. But if you were able to pick up the second ball and turn it upside down without stopping it spinning, it would then look exactly like the first one, as long as it is perfectly spherical. In other words, you cannot see any difference between the normal-time and reversed-time versions - they are the same. Now imagine the tennis balls are not completely spherical but have a small bulge on one side - in other words, they are slightly pear-shaped. This time, the two are not the same; if you turn one upside down, it will look quite different from the other: an asymmetry in space corresponds to an asymmetry in time. So, if the fundamental particles that make up matter are not spatially symmetrical, there will be a corresponding asymmetry under time reversal, and hence (if the CPT theorem is right) between matter and antimatter.

But what do we know of the physical structure of a tiny sub-atomic particle? Well, we believe that protons and neutrons are each made up of three charged quarks, in such a way that the net charge of a proton is +1 (in terms of the electron charge) and that of the neutron is zero. Imagine these quarks orbiting one another, as if the particle were a miniature atom. If the "orbitals" look symmetrical, so will the overall particle be symmetrical. But they might not be; in the neutron, for instance, there could be a finite distance between the average positions of the positively charged up quark and the negatively charged down quarks; and this spatial asymmetry, which would correspond to an asymmetry under time reversal if the motions of the quarks were all reversed, and therefore a matter-antimatter asymmetry, would also have a tell-tale signature - an electric dipole moment.

An electric dipole moment (EDM) is a pair of equal and opposite charges (±q) separated by a distance x, and numerically it is the product of the two, or qx. (The net charge of the system is zero, of course). It is this quantity that physicists are looking for, in order to find evidence of the asymmetry between matter and antimatter.

In fact all current theories - including the generally accepted Standard Model of particle physics - predict that fundamental particles will have EDMs, but the predicted values are numerically extremely small - for instance, the Standard Model predicts a separation of the order of 10^-20 of the diameter of the particle. So how can we detect them? Well, a particle with a non-zero EDM - even an electrically neutral particle like the neutron - will experience a torque when placed in an electric field. The effect of a torque on a spinning object like a neutron is to make its axis of rotation precess, like the wobbling motion of a spinning top if its axis is moved out of the vertical. This precession is about an axis parallel to the field producing the torque - gravity in the case of the top, or the electric field in the case of the EDM; and the precession frequency depends only on the strengths of the field and of the EDM, given that the angular momentum of the spinning body is constant.

A neutron will also precess if it is placed in a magnetic field, because it has a magnetic dipole moment (like a tiny bar magnet) which of course arises from the fact that it is made up of spinning electric charges. A neutron in a region of parallel electric and magnetic fields will thus precess at a frequency that depends on a combination of these effects, and the way they combine depends on whether the fields are parallel or antiparallel. So, all you have to do is subject the neutron to parallel magnetic and electric fields, and then reverse the electric field and look for a change in the precession frequency, which will be evidence of a non-zero EDM.

Fig 1. The effect of the pulse is to rotate the spin vector clockwise in the plane of the paper. Here we show the effect of the second pulse if it is in phase with the neutron precession. If the spin vector in (3) were pointing to the left (corresponding to being 180° out of phase with the oscillating field) the pulse would flip it back into a "spin up" state.

But how do we measure this frequency, I hear you say? Well, we make use of a trick invented by American physicist Norman Ramsey, which won him the Nobel Prize in 1989. The Ramsey method of separated oscillating fields uses a short pulse of oscillating magnetic field to flip the spins of the neutrons - originally polarised so as to be parallel to the magnetic and electric fields - through 90°, so that they begin to precess around the fields. They are then simply left to precess for several minutes - a very long time in the life of a neutron - before a second pulse of oscillating magnetic field, exactly in phase with the first, is applied; finally we allow the neutrons to enter a detector through a polarising filter, which only allows through neutrons in the original "spin up" state. The probability that a neutron is in this state at the end of the experiment depends on where in its circular "orbit" the neutron's spin vector is when the second pulse is applied. If the precession frequency is equal to the field frequency, there is a high probability that it will "flip" in the same direction as last time, and hence will end up in the "spin down" state. (See Fig. 1). But if they are not the same, this probability is smaller, and there is a correspondingly bigger chance that the neutron will end up in the "spin up" state instead. If we plot the number in the original spin state against the frequency of the oscillating field, we get a resonance pattern with a minimum at the precession frequency, and when we reverse the electric field, the position of this minimum will shift by an amount proportional to the EDM.

What is not, perhaps, so obvious is how the neutrons can be contained for long enough for such a tiny change in the precession frequency to be noticeable. Neutrons emanating from a typical neutron source will simply pass through the walls of any container, or else be absorbed by them. What we need to do is to slow them down, because - if you choose the right material for your container - there is a threshold energy (the Fermi Potential) below which the neutrons simply cannot escape or even enter the walls - they just bounce off instead, and can therefore be kept inside the experiment for the required time. Such neutrons are called ultra-cold.

Ultra-cold neutrons (UCN) can be produced in a variety of ways. If you use a nuclear reactor as your neutron source, the neutrons, which are emitted as a result of the nuclear fission reactions taking place inside the reactor, will initially have energies of around 1MeV, corresponding to a speed of some 14 million metres per second. In a conventional thermal reactor this speed is reduced by a factor of around 1,000 by the reactor's moderator (usually a light material such as graphite or heavy water). But for our purposes we must slow them down much more than that - to speeds of just a few metres per second, in fact, or, in human terms, an average running pace. For this, we use a mechanical velocity selector to slow them down to an effective temperature of 11K - corresponding to a few hundred metres per second - and then immerse them in superfluid liquid helium at 0.5K. This combination of neutron and helium energies leads to an interaction in which the neutrons can lose nearly all their remaining energy, bringing them into the ultra-cold region where, since the process is not reversible, they remain until we let them out.

It is now simply a question of applying our magnetic and electric fields and waiting. After the desired time - long enough for any slight phase difference to become measurable, but not long enough for the neutrons' polarisation to be destroyed by the frequent collisions with the walls - we literally pull the plug. The neutrons are so slow that gravity, usually neglected in high energy experiments, now takes over and they simply drain out of the experimental chamber and through the neutron detector, like so much bathwater! The figure below shows a cross section through the experimental apparatus.

"We", by the way, is a small collaboration of physicists from the UK's Rutherford Appleton Laboratory and from Sussex and Oxford Universities, from Kure University in Hiroshima, Japan, and the Institut Laue-Langevin in Grenoble, France (where the experiment will actually take place). Previous experiments have shown that if the neutron has an EDM it can be no bigger than 10^-28 e metres (where e is the charge on the electron). These experiments used mechanical means to produce the ultra-cold neutrons - a much less efficient process than liquid helium - so that our neutron flux, which, with other factors, determines the sensitivity of the experiment, will be considerably larger, and we hope to push that limit down by a factor of 100 or so. Such a sensitive measurement requires careful preparation, of course; we are currently looking into such questions as exactly how our liquid helium will behave when subjected to electric fields of the order of millions of volts per metre, and how we can screen out fluctuations in the ambient magnetic field which would otherwise affect the precession frequency; and consequently the experiment is not likely to start taking data until at least 2006.

Previous experiments have disproved several theories which predicted larger values for the EDM, and next in the firing line is supersymmetry, which predicts a value of about 10^-29 e m. Even if we don't find out why we are here, we will at least cut down on the number of possible explanations.