ELECTRIC FROGS
Our story begins in the late 1700s with Galvani's invention of the battery, which was later improved by Volta, another Italian. Galvani's study of frog reflexes—he hung frog muscles on the latticework outside his window and watched them twitch during thunderstorms—demonstrated "animal electricity." This stimulated Volta's work about 1790, and a good thing too. Think of Henry Ford installing a box of frogs in each of his cars with instructions to the motorist: "Frogs must be fed every fifteen miles." What Volta found was that the frog electricity had to do with two dissimilar metals separated by some kind of frog goop, for Galvani's frogs were hung on brass hooks on an iron latticework. Volta was able to produce an electrical current sans frog by experimenting with different pairs of metals separated by pieces of leather (standing in for the frogs) soaked in brine. He soon created a "pile" of zinc and copper plates, realizing that the larger the pile, the more current he could drive through an external circuit. Crucial to this work was Volta's invention of an electrometer for measuring the current. This research yielded two important results: a laboratory tool for producing currents and a realization that electricity could be produced by chemical reactions.
Another important development was Coulomb's measurement of the strength and behavior of the electrical force between two charged balls. To make this measurement he invented the torsion balance, a device exquisitely sensitive to tiny forces. The force he was after, of course, was electricity. Using his torsion balance, Coulomb determined that the force between electrical charges varied inversely as the square of the distance between them. He also discovered that like sign charges (+ + or − −) repelled one another, whereas unlike charges (+ −) attracted. Coulomb's law, giving the F for electric charges, will play a crucial role in our understanding of the atom.
In a veritable frenzy of activity, there began a series of experiments on what scientists first believed to be the separate phenomena of electricity and magnetism. In a brief period of about fifty years (1820–1870) these experiments led to a grand synthesis that resulted in a unified theory that included not only electricity and magnetism but light as well.
SECRET OF THE CHEMICAL BOND: PARTICLES AGAIN
Much of our early knowledge of electricity emerged from discoveries in chemistry, specifically what is now called electrochemistry. Volta's battery taught scientists that an electrical current can flow around a circuit in a wire that reaches from one pole of the battery to the other. When the circuit is interrupted by attaching wires to pieces of metal immersed in a liquid, the current flows through the liquid. The current in the liquid, they found, creates a chemical process: decomposition. If the liquid is water; hydrogen gas appears near one piece of metal, oxygen near the other. The proportion of 2 parts hydrogen to 1 part oxygen indicates that water is being decomposed into its constituents. A solution of sodium chloride would result in a plating of sodium on one "terminal" and the appearance of the greenish gas chlorine at the other. The industry of electroplating would soon emerge.
The decomposition of chemical compounds by an electrical current indicated something profound: a connection between atomic binding and electric forces. The notion gained currency that the attractions between atoms—that is, the "affinity" one chemical has for another—were electrical in nature.
Michael Faraday began his work in electrochemistry by systematizing the nomenclature. As with Lavoisier's naming of chemicals, this helped a lot. Faraday called the metals immersed in the liquid "electrodes." The negative electrode was a "cathode," the positive an "anode." When the electricity zipped through the water, it impelled a migration of charged atoms through the liquid from cathode to anode. Normally, chemical atoms are neutral, having neither a positive nor a negative charge. But the electric current somehow charged the atoms. Faraday called these charged atoms "ions." Today we know that an ion is an atom that has become charged because it has lost or gained one or more electrons. In Faraday's time, they didn't know about electrons. They didn't know what electricity was. But did Faraday suspect the existence of electrons? In the 1830s he carried out a series of spectacular experiments that resulted in two simple summary statements known as Faraday's laws of electrolysis:
The mass of chemical released at an electrode is proportional to the current multiplied by the length of time it runs. That is, the released mass is proportional to the amount of electricity that passes through the liquid.
The mass liberated by a fixed quantity of electricity is proportional to the atomic weight of the substance multiplied by the number of atoms in the compound.
What these laws mean is that electricity is not smooth and continuous but can be divided into "chunks." Given Dalton's idea of atoms, Faraday's laws tell us that atoms in the liquid (ions) migrate to the electrode, where each ion is presented with a unit quantity of electricity that converts it to a free atom of hydrogen, oxygen, silver, or whatever. The Faraday laws thus point to an unavoidable conclusion: there are particles of electricity. This conclusion, however had to wait about sixty years to be dramatically confirmed by the discovery of the electron at the end of the century.
A SHOCK IN COPENHAGEN
To continue the history of electricity—the stuff that appears in two or three slots in your electrical outlets, for a price—we have to go to Copenhagen, Denmark. In 1820 Hans Christian Oersted made a key discovery—some historians claim that it is the key discovery. He created an electric current in the approved manner, with wires connecting one terminal of a Voltaic contraption (battery) to the other. Electricity was still a mystery, but an electric current involved something called electric charge, moving through a wire. No surprise there, until Oersted placed a compass needle (a magnet) near the circuit. When the current flowed, the compass needle veered from pointing to the geographic North Pole (its normal job description) to taking a funny position at right angles to the wire. Oersted worried about this effect until it dawned on him that, after all, a compass is designed to detect magnetic fields. So the current in the wire must be producing a magnetic field, no? Oersted had discovered a connection between electricity and magnetism: currents produce magnetic fields. Magnets of course also produce magnetic fields, and their ability to attract pieces of iron (or to hold snapshots on refrigerator doors) was well studied. The news traveled across Europe and created a great stir.
Running with this information, the Parisian André Marie Ampère found a mathematical relation between current and a magnetic field. The detailed strength and direction of this field depended on the current flowing and on the shape (straight, circular, or whatever) of the wire carrying the current. By a combination of mathematical reasoning and many experiments hastily carried out, Ampère generated a one-man storm of controversy out of which emerged, in the fullness of time, a prescription for calculating the magnetic field produced by an electric current through any configuration of wire—straight, bent, formed into a circular loop, or wound densely on a cylindrical form. Since current passed through two straight wires produces two magnetic fields, these fields can push on each other; effectively, the wires exert force on each other. This discovery made possible Faraday's invention of the electric motor. The fact that a circular loop of current produces a magnetic field was also profound. Could it be that what the ancients called lodestones, natural magnets, actually are composed of atomic-scale circular currents? Another clue to the electrical nature of atoms.
Oersted, like so many other scientists, felt driven toward unification, simplification, reduction. He believed that gravity, electricity, and magnetism were all different manifestations of a single force, which is why his discovery of a direct connection between two of these forces was so exciting (shocking?). Ampère, too, looked for simplicity; he essentially tried to eliminate magnetism by considering it an aspect of electricity in motion (electrodynamics).
DÉJÀ VU ALL OVER AGAIN
Enter Michael Faraday (1791–1867). (Okay, he has already entered, but this is his formal intro. Fanfare, please.) If Faraday w
as not the greatest experimenter of his time, he is certainly a candidate for that tide. It is said that he has generated more biographies than Newton, Einstein, or Marilyn Monroe. Why? Partly it's the Cinderella aspect of his career. Born into poverty, at times hungry (he was once given a loaf of bread as his only food for a week), Faraday was practically unschooled, with a strong religious upbringing. Apprenticed to a bookbinder at the age of fourteen, he actually managed to read some of the books he bound. He thus educated himself while developing a manual dexterity that would serve him well as an experimenter. One day a client brought in a copy of the third edition of the Encyclopaedia Britannica to be rebound. It had an article on electricity. Faraday read it, was hooked, and the world changed.
Think about this. Two items are received by the network news offices over the AP wires:
FARADAY DISCOVERS ELECTRICITY, ROYAL SOCIETY LAUDS FEAT
and
NAPOLEON ESCAPES FROM ST. HELENA,
CONTINENTAL ARMIES ON THE MARCH
Which item makes the six o'clock news? Right! Napoleon. But over the next fifty years Faraday's discovery literally electrified England and set in motion as radical a change in the way people live on this planet as has ever flowed from the inventions of one human being. Now if only the gatekeepers of TV journalism had been forced to satisfy a real science requirement in college...
CANDLES, MOTORS, DYNAMOS
Here is what Michael Faraday accomplished. Starting his professional life as a chemist at the age of twenty-one, he discovered a number of organic compounds, including benzene. He made the transition to physics by cleaning up electrochemistry. (If those University of Utah chemists who thought they had discovered cold fusion in 1989 had understood Faraday's laws of electrolysis better, perhaps they would never have embarrassed themselves as well as the rest of us.) Faraday then went on to make major discoveries in the fields of electricity and magnetism. He:
discovered the law (named for him) of induction, whereby a changing magnetic field creates an electric field
was the first to produce an electric current from a magnetic field
invented the electric motor and dynamo
demonstrated the relation between electricity and chemical bonding
discovered the effect of magnetism on light
and much more!
All this without a Ph.D., M.A., B.A., or high school equivalency degree! He was also mathematically illiterate. He wrote up his discoveries not in equations but in plain descriptive language, often accompanied by pictures to explain the data.
In 1990 the University of Chicago launched a TV series called The Christmas Lectures, and I was honored to give the first one. I called it "The Candle and the Universe," borrowing the idea from Faraday, who started the original Christmas lectures for children in 1826. In his first talk he argued that all known scientific processes were illustrated by the burning candle. This was true in 1826, but by 1990 we had learned about a lot of processes that do not take place in the candle because the temperature is too low. Nevertheless, Faraday's lectures on the candle were lucid and entertaining and would make a great Christmas present for your children if some silver-voiced actor would make some CD's. So add another facet to this remarkable man—Faraday as popularizer.
We have already discussed his electrolysis research, which prepared the way for the discovery of the electrical structure of chemical atoms and, indeed, for the existence of the electron. Now I want to describe Faraday's two most remarkable contributions: electromagnetic induction and his almost mystical concept of "field."
The route to the modern understanding of electricity (more properly, electromagnetism or the electromagnetic field) is akin to the famous baseball double-play combination linker to Evers to Chance. In this case it's Oersted to Ampère to Faraday. Oersted and Ampère made the first steps in understanding electric currents and magnetic fields. Electric currents flowing in wires, like those in your house, make magnetic fields. Thus you can make as powerful a magnet as you want, from the tiny battery-operated magnets that drive small fans to the giant ones used in particle accelerators, by organizing currents. This understanding of electromagnets illuminates our understanding of natural magnets as containing atomic-scale current elements that cooperate to generate a magnet. Nonmagnetic materials also have these Amperian atomic currents, but their random orientation produces no net magnetism.
Faraday struggled for a long time to unify electricity and magnetism. If electricity can make magnetic fields, he wondered, can magnets make electricity? Why not? Nature loves symmetry. But it took him more than ten years (1820–1831) to prove it. This was probably his greatest achievement.
Faraday's experimental discovery is called electromagnetic induction, and the symmetry he sought emerged in a surprising form. The road to fame is paved with good inventions. Faraday first wondered whether a magnet could make a current-carrying wire move. Visualizing the forces, he rigged up a device that consisted of a wire connected to a battery at one end, with the other end hanging in a beaker of mercury. The electric wire hung free so that it could revolve around an iron magnet in the beaker. When the current was turned on, the wire moved in a circle around the magnet. We know this odd invention today as an electric motor. Faraday had converted electricity to motion, which could do work.
Let's jump to 1831 and another invention. Faraday wrapped a large number of turns of copper wire on one side of a soft iron doughnut, then connected the two ends of the coil to a sensitive current-measuring device called a galvanometer. He wrapped a similar length of wire on the other side of the doughnut, connecting these ends to a battery so that current could flow in the coil. This device is now called a transformer. Let's review. We have two coils wound on opposite sides of a doughnut. One, let's call it A, is connected to a battery; the other (B) is connected to a galvanometer. What happens when you turn on the juice?
The answer is important to the history of science. When the current flows in coil A, the electricity produces magnetism. Faraday reasoned that this magnetism should induce a current in coil B. But instead he got a strange effect. When he turned on the current, the needle in the galvanometer connected to coil B deflected—voila! electricity!—but only momentarily. After the sudden jump, the needle remained pointed maddeningly to zero. When he disconnected the battery, the needle deflected briefly in the opposite direction. Increasing the sensitivity of the galvanometer had no effect. Increasing the number of turns in each coil had no effect. Using a much stronger battery had no effect. And then the Eureka moment (in England it is called the By Jove moment): Faraday figured out that current in the first coil had induced a current in the second, but only when the first current was changing. So, as the next thirty years or so of research showed, a changing magnetic field generates an electric field.
The technology that emerged in due course was the electric generator. By rotating a magnet mechanically, one can produce a constantly changing magnetic field, which will generate an electric field and, if connected to a circuit, an electric current. One can rotate a magnet by turning a crank, by using the force of a waterfall, or by harnessing a steam turbine. Now we had a way of generating electricity to turn night into day and to energize those electrical outlets in home and factory.
But we pure scientists ... we are on the track of the a-tom and the God Particle; we dwell on the technology only because it would have been awfully hard to make particle accelerators without Faraday's electricity. As for Faraday, he probably wouldn't have been impressed with the electrification of the world except that now he could work at night.
Faraday built the first hand-cranked electrical generator himself; it was called a dynamo in those days. But he was too involved in the "discovery of new facts ... being assured that the latter [practical applications] would find their full development hereafter" to figure out what to do with it. The story is often told that the British prime minister visited Faraday's laboratory in 1832 and, pointing to the funny machine, asked what use it was. "I know not
, but I wager that one day your government will tax it," said Faraday. A tax on electrical generation was levied in England in 1880.
THE FIELD BE WITH YOU
Faraday's major conceptual contribution, crucial to our history of reductionism, was the field. To prepare for this, we must go back to Roger Boscovich, who published a radical hypothesis some seventy years before Faraday's time, carrying the a-tom an important step forward. How do a-toms collide? he asked. When billiard balls collide, they deform; their elastic recovery pushes the balls apart. But a-toms? Can one imagine a deformed a-tom? What would deform? What recover? Boscovich was led by such thinking to reduce a-toms to a dimensionless, structureless mathematical point. This point is the source of forces, both attractive and repulsive. He constructed a detailed geometric model that treated atomic collisions very plausibly. The point a-tom did everything that Newton's "hard, massy atom" did but offered advantages. Although it had no extension, it did have inertia (mass). Boscovich's a-tom reached out into space via forces radiating from it. This is an extremely prescient concept. Faraday also was convinced that a-toms were points, but since he could not offer proof, his support was muted. The Boscovich/Faraday view was this: matter consists of point a-toms surrounded by forces. Newton had said force acts on mass, so this was clearly an extension of his idea. How does this force manifest itself?
"Lets play a game," I say to the students in a large lecture hall. "When the student to your left lowers his hand, you raise and lower your hand." At the end of the row we pass the signal up one row and switch to "student on your right." We begin with the student at the extreme left of the front row, who raises her hand. Soon the "hand-up" wave travels across the room, up, back across, and so on until it peters out at the top of the hall. What we have is a disturbance propagating with some speed through a medium of students. It's the same principle as the wave, seen in football stadiums across the land. A water wave has the same properties. Although the disturbance propagates, the water particles stay put, bobbing up and down but not involved in the horizontal velocity of the disturbance. The "disturbance" is the height of the wave. The medium is water. The velocity depends on the properties of water. Sounds propagate through air in much the same way. But how does a force reach out from atom to atom through intervening empty space? Newton punted. "I frame no hypothesis," he said. Framed or not, the common hypothesis for how a force propagate was the mysterious action-at-a-distance, a kind of placeholder for a future understanding of how gravity is supposed to work.
The God Particle: If the Universe Is the Answer, What Is the Question? Page 16
