MORE INFO

www.jlab.org/FEL/ for the Jefferson Lab's Free-Electron Laser.
www.aip.org/history/electron/ for the American Institute of Physics' history of the electron.
www.iop.org/Physics/Electron/Exhibition produced in collaboration between the Science Museum in London, and the Institute of Physics. The site offers an Internet movie showing a modern account of the discovery of the electron as well as interactive animation of a Thomson experiment. Visitors can listen to a recording of Thomson describing his discovery.
web.lemoyne.edu/~giunta/jthomson.html Thomson's 1906 Nobel Prize lecture.

J.J. THOMSON


Joseph John (J.J. to his friends and colleagues) Thomson was born on Dec. 18, 1856, near Manchester, England.

He attended Owens College in Manchester, where his professor of mathematics encouraged him to apply for a scholarship at Trinity College, one of the most prestigious of the colleges at Cambridge University.



Fred Dylla holds a cathode ray tube similar to the one used by J.J. Thomson in his initial experiments that led to his discovery of the electron.
ALEXA WELCH EDLUND/TIMES-DISPATCH


Thomson won the scholarship, and in 1880 finished second in his class in the graduation examination in mathematics. Trinity gave him a fellowship, and he stayed on there, trying to determine the nature of atoms and electromagnetic forces.

In 1884, Thomson was chosen to be the third Cavendish professor at Cambridge. He was an inexperienced and clumsy experimenter, but he presided over the flowering of experimental physics at the Cavendish Laboratory.

He trained many outstanding physicists, including seven Nobel Prize winners and 27 Fellows of the Royal Society. Among them was Thomson's son, George Paget Thomson, who became a prominent physicist himself.

In 1906, J.J. Thomson was awarded the Nobel Prize in physics for his research into the discharge of electricity in gases. In 1918 he was chosen master of his old college, Trinity, and the next year he resigned the Cavendish professorship. He continued to work at Trinity until shortly before his death on Aug. 30, 1940.

'Of whose laws we are ignorant'

The opening statement from Joseph John Thomson's seminal article on "corpuscles," or electrons, in Philosophical Magazine, published in August 1897. Portions of Thomson's experiments had been announced earlier that year.

"The experiments discussed in this paper were undertaken in the hope of gaining some information as to the nature of the Cathode Rays. The most diverse opinions are held as to these rays; according to the almost unanimous opinion of German physicists they are due to some process in the aether to which — inasmuch as in a uniform magnetic field their course is circular and not rectilinear — no phenomenon hitherto observed is analogous: another view of these rays is that, so far from being wholly aetherial, they are in fact wholly material, and that they mark the paths of particles of matter charged with negative electricity. It would seem at first sight that it ought not to be difficult to discriminate between views so different, yet experience shows that this is not the case, as amongst the physicists who have most deeply studied the subject can be found supporters of either theory.

"The electrified-particle theory has for purposes of research a great advantage over the aetherial theory, since it is definite and its consequences can be predicted; with the aetherial theory it is impossible to predict what will happen under any given circumstances, as on this theory we are dealing with hitherto unobserved phenomena in the aether, of whose laws we are ignorant."

Happy birthday, Dear Electron

Jefferson Lab marks particle's discovery

The guests snacked on cake and oohed and ahhed over the host's party tricks with electricity and vacuum tubes. They even listened attentively to an hour-long toast on the electron's 100th birthday.

Friends of the electron gathered at the Thomas Jefferson National Accelerator Facility last month in honor of the electron's discovery by Joseph John Thomson, a physicist at Cambridge University. Physicist Fred Dylla was the host of the gathering, spinning tales of dead scientists and describing the series of experiments that led to the electron's detection in 1897.

Dr. Dylla reminded the guests that without the electron, most of the 20th-century's inventions — and their lab — wouldn't exist.

"Thomson really launched the 20th-century technology that took advantage of the electron," Dylla said. "What came out of those primitive tubes was the birth of modern electronics."

Today, the electron is known as one of the two fundamental particles (the other is the quark). Scientists use both in the Jefferson Lab's $600 million particle accelerator, which aims hair-thin beams of electrons at various atoms to learn more about quarks, which make up protons and neutrons.

The path to quarks and 20th-century particle physics began in the late 1890s with a rather simple and relatively cheap device called a cathode ray tube. Made of glass and bits of metal, it hardly impresses someone used to computers, CD players, supermarket scanners and televisions as the original, figurative atom-splitter.

A cathode ray tube consists of a thin glass tube with metal electrodes at each end. A pump removes the air in the tube, creating a vacuum. When an electric current passes through the tube, a luminous spot appears on the wall of the tube opposite the negative electrode, known as the cathode.

Nineteenth-century scientists who placed objects in the tube found the objects cast shadows, showing that a stream of rays emanated from the cathode. These emanations were called cathode rays.

The CRT (as we now call it) shattered the conviction that the atom was like a tiny billiard ball, indivisible and chargeless. The notion was so entrenched that when Thomson first detected a charge running through the tube, he thought it was a mistake, not a discovery that would win him the Nobel Prize just nine years hence.

In the late 19th century, competition to determine the components of cathode rays was fierce, involving the best minds of the day. Thomson got there first in part because he had better equipment. The electron's discovery also took a mind that could recognize what was happening in the cathode ray tube.

"His work was really a superb example of the interplay between science and technology," Dylla said.

Thomson found that by using magnets and electrical fields, he could bend or displace the rays toward the positive electrode, proving that the rays were made of negatively charged particles. He called these particles "corpuscles." Scientists later renamed them "electrons."

"I can see no escape from the conclusion that they are charges of negative electricity carried by particles of matter," Thomson wrote. "The question next arises, [w]hat are these particles? [A]re they atoms, or molecules, or matter in a still finer state of subdivision?"

The answer, Thomson went on to find, was that the corpuscles had a mass about 1,700 times smaller than the hydrogen atom, the smallest thing then known, and were the stuff of which atoms were made.

"We have in the cathode rays matter in a new state, a state in which the subdivision of matter is carried very much further than in the ordinary gaseous state: a state in which all matter . . . is of one and the same kind; this matter being the substance from which all the chemical elements are built up," he wrote.

That these corpuscles were a fundamental part of the atom, and of all matter, was a leap of scientific faith. Thomson himself recognized this:

"At first there were very few who believed in the existence of these bodies smaller than atoms. I was even told long afterwards by a distinguished physicist who had been present at my [1897] lecture at the Royal Institution [announcing his discovery] that he thought I had been 'pulling their legs.' "

Thomson's measurements couldn't be ignored, or otherwise explained, said Dr. John Fenn, research professor of chemistry at Virginia Commonwealth University, whose own field of study, mass spectrometry, arose directly from Thomson's work.

"His clever experiments . . . forced other people to think that there were subatomic particles," Fenn said. "He realized that what he saw was significant and went on to do the quantitative work that made it that much more convincing to his scientific colleagues."

"It was a revolution," said Dr. George Sanzone of Virginia Tech, a self-described fan of Thomson. "He was the starting genius" of today's quantum physics.

Once other scientists were convinced, Thomson's discovery opened up the possibility of other subatomic particles. For Thomson, however, the electron was an end in and of itself. He and his colleagues at Cambridge's Cavendish Laboratory used to laud the fundamental particle with the toast: "The electron: may it never be of use to anyone."

Dylla's not sure if the toast was in jest or not. Either way, he said, "It is ironic that the discovery of the electron turned out to have launched two revolutions, one in science and one in technology."

For all the debt to Thomson, it's the technological revolution and what electrons can do that interest Dylla and his colleagues at the Jefferson Lab.

Researchers at the Newport News facility are building one of the CRT's direct descendants, the Free-Electron Laser, which uses the electron to produce one of the most powerful and controllable light sources ever. When the FEL at Jefferson Lab comes online next spring, it will be about a million times more powerful than supermarket laser scanners.

The commercial potential for the Jefferson Lab FEL has corporations such as DuPont and Northrup Grumman so interested that they are helping the federal and state governments fund the laser's $34.2 million price tag. Potential uses for the laser include making food packaging resistant to bacteria, improving the speed of airplanes and preventing metal corrosion.

For all its possible uses, only 16 FELs have been built since the laser was invented in 1971. A dozen others, including the one at Jefferson Lab, are under construction. The laser group at Jefferson Lab won financial support from governmental and industrial sources in part because its FEL uses some of the same components as the lab's particle accelerator, helping to keep costs down.

The laser's name refers to the electrons' ability to move freely inside the laser's vacuum, where they can move without resistance from air molecules that could slow or alter their course.

Light production begins at the laser's injector, located underneath a two-story building not far from the particle accelerator. An electron gun shoots 40 million bunches of electrons per second into a 60-foot-long column.

The column is chilled in a superconducting chamber to minus 456 degrees and revs up the electrons to 40 million volts each before spewing them out at nearly the speed of light.

These supercharged electrons are then "wiggled" by a series of magnets so that they lose some of their energy. That energy is released as extremely intense light.

The laser beam, initially about a half-inch in diameter, is reflected out of its underground chamber and into any two of the six laboratories in the two-story building. Mirrors along the beam's path can change the size of the beam and magnets can change its wavelength.

The project will start with infrared light, but in a few years, Dylla said, the team hopes eventually to produce the more powerful ultraviolet light.

First light is expected in May.