by T. R. Reid
“J.J. spent a good part of most days sitting in the armchair that had belonged to Maxwell, doing mathematics,” a student recalled. He sat there thinking, working out problems, because he had absolute confidence in the power of thought. He was certain that the human mind, aided by mathematics, could comprehend all physical phenomena. But the professor’s wide-ranging curiosity reached far beyond math and physics. His workday ran, at the most, from ten in the morning to six at night (with a break for afternoon tea), and he regularly found time to cheer for Cambridge at crew and rugby matches. He read A. E. Housman’s poetry in manuscript and made it a point never to miss opening night of a new Gilbert and Sullivan operetta. He was fascinated by gardening, golf, and American politics. He was British to the core. In his memoirs he notes with great pride that twenty-seven of his students (including his son) were elected to the Royal Academy; as an aside, he mentions that seven of them (including his son) also picked up Nobel Prizes. J.J.’s own Nobel Prize, in 1906, seems to have satisfied him less than the knighthood he received two years later. When he died, at eighty-four, in 1940, he was buried in Westminster Abbey near the grave of Isaac Newton.
Physicists talk about Thomson’s cathode ray experiment of 1897 the way architects talk about the Guggenheim Museum in Bilbao. The approach was elegant, the conclusion bold and stunning, and the result left an indelible mark on everything that came after. Thomson was determined to find out everything he could about that electric beam shooting through the cathode ray tube. Where did it come from? What was it made of? Was that beam of pure energy a wave, or a stream of particles? Thomson decided he could ascertain the nature of the electric beam by subjecting it to different forces and measuring the results as precisely as possible. His experiments were beautiful to watch. By placing magnets around the glass tube, Thomson could make the cathode ray spin in a perfect spiral. Since the vacuum in the tube was not complete, the cathode ray would occasionally strike a stray gas atom and give it an electric charge. The charged atom, or ion, would glow with a colorful splendor until it was neutralized, or recombined, by another atom. So lovely were the results that Thomson and his colleagues wrote a song about it all, to the tune of “Clementine”:
In the dusty lab’ratory,
’Mid the coils and wax and twine,
There the atoms in their glory
Ionize and recombine.
Chorus: Oh my darlings! Oh my darlings!
Oh my darling ions mine!
You are lost and gone forever
When just once you recombine.
In the weird magnetic circuit
See how lovingly they twine,
As each ion describes a spiral
Round its own magnetic line.
Chorus
In a tube quite electrodeless,
They discharge around a line,
And the glow they leave behind them
Is quite corking for a time.
Chorus
At first, Thomson’s research produced nothing but confusion; his findings did not seem to fit any single theory. The fact that the electric beam could be bent by a magnet or an electric field strongly suggested that it consisted of particles, for no wave was susceptible to magnetic attraction. But there was also compelling evidence against the particle hypothesis. There was no indication, even with the most exacting measurements, that the beam was deflected by gravity, as any stream of particles should be. Further, the beam could pass through a thin sheet of metal foil without leaving a hole. It was known that certain energy waves did that— just as light waves pass through a window without leaving a mark on the glass—but it seemed impossible for a solid particle to do so. To resolve these contradictory results, Thomson fell back on mathematics. Through a series of ingenious calculations, he determined the velocity of the moving beam—it traveled about 20,000 miles per second, infinitely faster than any object had ever been found to move before. Using that result, he calculated what the mass of the cathode ray particles would be, if indeed they were particles. This produced a result that was simply impossible. The mass of each particle would be less than one one-thousandth the size of a hydrogen atom. As a physical matter, this could not be right, for the hydrogen atom was well known to be the smallest of all material objects, and quite indivisible. As a mathematical matter, though, it could not be wrong. At this point, Thomson ended his experiments. There was nothing left to do but sit down in Maxwell’s armchair and think the matter through.
The conclusion Thomson drew from this research is so elementary today that it is almost impossible to appreciate the enormous intuitive leap, the sheer revolutionary daring, that was required to state it in 1897. In one fell swoop, he split the unsplit-table atom and explained the inexplicable force of electricity. He presented his wholly new picture of the physical world in an informal speech to the Royal Institution on a Friday evening in April.
“I have lately made some experiments which are interesting,” he began. Contrary to accepted scientific wisdom, he went on, the atom was not indivisible. He had found within the atom a new kind of particle—a particle at least a thousand times smaller than any atom. These subatomic particles—Thomson called them “corpuscles,” but eventually the term “electrons” was adopted instead—are universal constituents of all matter, found in every atom. The electrons are magnetic and carry a negative charge, which explains why a magnet or an electric field made them swerve from their straight path. Their mass is so minute that the force of gravity upon them was undetectable. They are so much smaller than any atom that they could slip through the open spaces in an atom of metal—and thus shoot through a sheet of metal foil without a trace. The cathode ray—for that matter, any electric current—consists of a stream of these charged particles.
In subsequent lectures and papers, Thomson and his colleagues refined their picture of the electron and postulated other subatomic particles. Since the electrons carried a negative charge, there must be another, positively charged particle in the atom; equal numbers of electrons and protons (as the positive particles came to be called) would make the ordinary atom electrically neutral. Because of its small mass, the electron can move about. Indeed, if a piece of metal is given an electric charge or heated to incandescence (as in the Edison Effect), electrons will stream away in huge numbers. This stream of moving electrons, Thomson concluded, is an electric current.
As with any shocking discovery, J. J. Thomson’s conclusion that the atom consisted of many smaller parts drew resistance at first. Fairly soon, though, the world of physics grew to accept the existence of the electron and the proton because the theory worked. It explained all sorts of things about atomic structure and about electricity that had not been understood before. It would eventually make possible the whole new world of semiconductor physics. It was a seminal idea.
The cream yellow laboratory where this idea was born still stands on the Cambridge campus, although nowadays it houses political scientists and economists. The university has built a fancy new Cavendish Laboratory for physicists and engineers a fair distance away. But at least the trustees of Cambridge have a sense of history. They marked the 100th anniversary of Thomson’s great breakthrough by mounting a celebratory plaque on the wall, outside his old lab:
Here in 1897 at the old Cavendish Laboratory
J.J. THOMSON
discovered the electron
subsequently recognized as the
first fundamental particle of physics
and the basis of chemical bonding, electronics, and computing.
Although Thomson’s leap of insight developed from experiments on electric current in a vacuum, his explanation of electric current—a flow of moving charges—is equally valid for current in a block of semiconductor material. If scientists had gone back to the semiconductor work that had largely been laid aside a generation earlier, the next important developments might have come in that field. As it happened, though, Thomson’s discovery was first applied in vacuum tubes—and the tube became the cent
ral component of electronic devices for the next half century. This came about because of the work of John Ambrose Fleming and Lee De Forest.
J. A. Fleming was a contemporary of Thomson’s. Like Thomson, he studied physics at college only because his family could not pay for an engineering apprenticeship. Like Thomson, Fleming was a distinguished professor (at University College, London) and a member of the leading scientific societies. Unlike Thomson, Fleming was interested in making money from his work; one result was that, like Edison, he became ensnarled in endless patent litigation. In pursuit of an income, Fleming worked the lecture circuit, traveling all over the British Isles to give scientific demonstrations. Hard-working, highly disciplined, extremely demanding of himself and those around him, Fleming was determined that everything about his lectures should be perfect—he rehearsed with a stopwatch so that every word and gesture would come at the right second—and thus failed to see the humor in a prank perpetrated during a widely advertised talk he gave in 1903. To demonstrate the wonders of wireless telegraphy, Fleming had arranged to receive a long-distance message in mid-lecture from the great Guglielmo Marconi himself. At the appropriate time, the telegraph key began to rattle, but instead of the weighty words Fleming had chosen to mark the historic occasion, the message was an off-color limerick (“There was a young fellow of Italy/ Who diddled the public quite prettily . . .”). It turned out that a playful student in the audience, a precursor of the modern hacker, had brought a transmitter of his own for purposes of mischief and broken into the network. Fleming, thoroughly unamused, wrote a thundering letter to The Times to denounce such “scientific hooliganism.”
Fleming’s interest in money also led him to pioneer a practice common among modern faculty members—consulting to industrial concerns. In 1882 he was appointed “electrician” (the modern term would be “science advisor”) to the Edison Electric Light Company, Ltd. He held that position only a few years, but they happened to be the years when Edison and Upton were working on the Edison Effect. Fleming, as we have seen, made the interesting discovery in 1884 that the Edison Effect current—the current from the filament to the metal plate—never changed direction, even when alternating current was sent through the filament. Years later, after Thomson had established that the current is a flow of electrons, Fleming was able to explain why. Electrons boiling off the hot filament flowed to the metal plate. But the plate was not hot enough to emit electrons, so no current flowed back from plate to filament. Thus the Edison Effect always produced direct current.
At the turn of the century Fleming landed another consulting position, this time with Marconi’s Wireless Telegraph Company, Ltd. Radio in this primordial era was still as much a toy as a tool, and there were several problems facing the Marconi firm. One was the inability to tune the radios to a specific frequency, an improvement that could have prevented pranksters from sending limericks in place of important messages. A more significant obstacle to serious radio transmission was the absence of a reliable rectifier. A radio transmitter beams out signals that travel through the sky in the form of alternating current. But the receiving instruments that turn those signals into information—a telegraph key, for example, or a radio’s speaker—operate on direct current. The crucial need, then—the missing link—was a device that could take the alternating current sent by the transmitter and convert, or rectify, it to a direct current that echoed the pulsations in the original signal.
The materials known today as semiconductors have this rectifying quality, and the first radios employed a semiconductor crystal to rectify the signal. Such radios came to be known as crystal sets. Since nobody knew much about semiconductor technology, crystal sets seemed to work only when they chose to, and were too capricious for business use. If radio was to have any serious impact on the world, someone would have to find a dependable way to convert the alternating radio signal into direct current. This was the task Marconi assigned to J. A. Fleming.
Fleming initially tried to get reliable rectifying action out of the standard crystal. In October 1904, however, he realized that this approach would not be fruitful and began thinking hard about other devices that could convert an alternating current to direct current. What mechanism always produced direct current? Suddenly, he recalled his experiments of twenty years before. Fleming himself described the moment of discovery:
I was pondering on the difficulties of the problem when my thoughts recurred to my experiments in connection with the Edison Effect.
“Why not try the lamps?” I thought.
I went to a cabinet and brought out some lamps I had used in my previous investigations. . . . I started the oscillations in the primary circuit. To my delight I saw the needle of the galvanometer indicate a steady direct current. . . . We had in this particular kind of electric lamp a solution to the problem of rectifying high frequency wireless currents. The missing link in wireless was found—and it was an electric lamp.
In addition to its usefulness for radio, Fleming noted another important characteristic: the current flowing from filament to plate could switch off and on far more rapidly than any mechanical switch. “So nimble are these little electrons,” Fleming wrote, “that however rapidly we change the electrification . . . the plate current is correspondingly altered, even at the rate of a million times per second.” Because of this capacity to turn current on and off like a faucet, Fleming called the modified light bulb a “valve.” In the technical literature, Fleming’s rectifying lamp is called a diode, because it has two electrodes—the filament and the plate.
The Fleming diode made possible the production of dependable radio receivers. But there was still another problem to be resolved before radio became a practical instrument for sending information over any appreciable distance. Radio beams attenuate as they travel; the farther a signal has to go, the weaker it gets. After a signal had traveled 30 miles or so it was too weak to drive any kind of microphone; a slightly longer distance so diminished the current that it could barely move a telegraph key. What was needed was a device in the receiver that could strengthen, or amplify, the incoming signal without distorting its pattern of pulsation and modulation. The need was met, two years after Fleming’s invention, in the cluttered New York office of Lee De Forest.
De Forest was the son of a Congregationalist minister who moved, shortly after the Civil War, from the Midwest to Talladega, Alabama, to run the Negro college there. Stuck with this double whammy—a Yankee who lived with the Negroes—young Lee had few friends among his fellow whites in Talladega and spent his childhood reading science. He did indeed become a scientist; he took a Ph.D. at Yale after writing what was probably the first American dissertation on radio waves. Still, there was considerably more of Edison in him than of Thomson. A tireless self-promoter, De Forest worked feverishly all his life to attain the wealth and fame he felt he deserved. It was often an uphill battle. De Forest spent huge sums, with indifferent results, in legal battles over patents. He spent two years at the height of his career, 1912–13, fighting a federal mail fraud indictment resulting from his prediction, in a letter to potential investors, that the human voice would someday be broadcast across the Atlantic. No one could possibly believe such “absurd and deliberately misleading statements,” the prosecutor declared. The jury did, and De Forest was acquitted.
Not a shrinking violet, De Forest yielded to none in his esteem for his own scientific accomplishments. Asked to discuss his radio amplifier before the Franklin Institute, the inventor assured the assembled engineers that “a more revolutionary step was never taken in the history of engineering.” De Forest titled his autobiography Father of Radio, an immodesty that prompted Isaac Asimov to note that “few inventions have had so many fathers.” Just before he underwent delicate cancer surgery at the age of eighty-five, De Forest overheard the doctors saying the tumor would be removed by electrodesiccation. “Commonly known as the hot wire,” De Forest croaked from the operating table. “I invented it in 1907.”
De Forest’
s most important invention, the radio amplifier, was based on a fundamental principle of electricity: unlike charges attract, and like charges repel. An object carrying a negative charge is attracted toward a positive charge just as a paper clip is pulled toward a magnet. In a rainstorm, when clouds and earth develop opposite charges, the attraction is strong enough to pull a lightning bolt of electrons across the gap. This principle also explains the paparazzi’s favorite phenomenon, static cling. As the starlet scoots over to get out of the limousine, the hem of her dress rubs some electrons off her nylon stockings. With these excess electrons, the dress acquires a negative charge; the stockings become positive. Negative hem clings to positive stocking—at about mid-thigh, if the photographers are lucky—and a thousand flashbulbs pop as she leaves the car.
Tinkering with some early rectifying tubes in 1906, De Forest put a wire screen between the filament and the metal plate. Normally, this did not affect the Edison Effect current because electrons flowed right through the open screen to the plate. But when De Forest put a negative charge on the wire screen, like charges repelled: the negative screen repelled the negatively charged electrons, and current flowing to the plate was sharply reduced. When he sent a positive charge to the screen, unlike charges attracted: the screen attracted electrons, and the current to the plate was increased.
Experiments showed that a small change in the charge on the metal screen caused a big change in the current flowing to the metal plate. More important, the variations in the Edison Effect current exactly mimicked variations in the current sent to the wire screen. Here, then, was a precise amplifying mechanism. If the weak current from a distant radio signal was sent to the wire screen, it shaped a much stronger current that precisely matched the fluctuations of the radio beam. This stronger current could drive a telegraph key or loudspeaker. De Forest called the metal screen a “grid” and filed a patent entitled “Device for Amplifying Feeble Electrical Currents.”