Thematic Essay: Physics, from Leonardo to Hertz

X

Michael Faraday

Born into poverty, Michael Faraday was unschooled but had a strong religious upbringing. Apprenticed to a bookbinder at the age of 14, 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, including a volume with an article on electricity. Faraday read it and was hooked, and the world was never the same.

Over the next 50 years Faraday's discoveries literally electrified England and set in motion as radical a change in the way people live as has ever resulted from the inventions of one human being.

Faraday accomplished an amazing amount in the way of science and invention. Starting his professional life as a chemist at the age of 21, he discovered a number of organic compounds, including benzene. He made the transition to physics by thoroughly exploring the principles of electrochemistry. Faraday then went on to make major discoveries in the fields of electricity and magnetism. He was the first to produce an electric current from a magnetic field. He invented the electric motor and dynamo, demonstrated the relation between electricity and chemical bonding, and discovered the effect of magnetism on light. Faraday accomplished all this without a Ph.D., M.A., B.A., or high school equivalency degree. He was also mathematically illiterate. Faraday recorded his discoveries not in equations but in plain descriptive language, often accompanied by pictures based on his mental images, which helped him explain the data.

Faraday began his work in electrochemistry by systemizing the nomenclature. He 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 caused charged atoms to migrate 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. Scientists later learned that an ion is an atom that has become charged because it has lost or gained one or more electrons. Although the existence of electrons was unknown in Faraday’s time, some evidence suggests that he suspected their existence. 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:

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What these laws mean is that electricity is not smooth and continuous but can be divided into “chunks.” Faraday’s laws tell us that atoms in the liquid (ions) migrate to the electrode, where each ion is presented with a particular quantity of electricity. The Faraday laws thus point to an unavoidable conclusion: There are particles of electricity. This conclusion, however, had to wait about 60 years to be dramatically confirmed by the discovery of the electron.

The route to the modern understanding of electricity is akin to a double play combination in baseball: in this case, Oersted to Ampere to Faraday. Oersted and Ampere 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.

Faraday struggled for a long time to unify electricity and magnetism. If electricity can make magnetic fields, he wondered, can magnets make electricity? Indeed, why not, he reasoned, since nature loves symmetry? It took Faraday more than ten years, from 1820 to1831, to prove that the process was indeed possible, and it was arguably his greatest achievement. Faraday's experimental discovery is called electromagnetic induction, and the symmetry he sought emerged in a surprising form.

Faraday wondered whether a magnet could move a current-carrying wire. Visualizing the forces, he rigged up a device in which one end of a wire was connected to a battery and the other end hung in a beaker of mercury, a liquid conductor that helped the current flow. The electric wire hung free so it could revolve around an iron magnet in the beaker. When he turned the current on, the wire moved in a circle around the magnet. Faraday had converted electricity to motion with this invention, which we know today as an electric motor.

In another experiment, Faraday wrapped a large number of turns of copper wire on one side of a soft iron doughnut, and 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.) Faraday now had two coils wound on opposite sides of a doughnut. One coil, call it coil A, is connected to a battery, while the other, coil B, is connected to a galvanometer. What happened when he turned 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, but only momentarily. After the sudden jump, the needle remained pointed maddeningly to zero. When Faraday disconnected the battery, the needle deflected briefly in the opposite direction, then again pointed at zero. Increasing the sensitivity of the galvanometer had no effect. Increasing the number of turns in each coil still had no effect. Even using a much stronger battery had no effect. And then what scientists call the “Eureka” moment came: Faraday figured out that a changing current in the first coil had induced a current in the second, but only when the first current was actively turning on or off—that is, in the process of changing. As soon as the current—and the surrounding magnetic field—became stable, the field stopped inducing a change in the second coil. As Faraday suspected, and as the next 30 years or so of research would demonstrate, a changing magnetic field generates an electric field.

The technology that eventually emerged from this discovery was the electric generator. By rotating a magnet mechanically, it is possible to 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 humankind had a potential way to generate electricity and turn night into day.

Faraday built the first hand-cranked electrical generator, which in those days was called a dynamo. But he was too involved in the process of experimenting and making new discoveries 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 had. 'I know not, but I wager that one day your government will tax it,' said Faraday. Sure enough, a tax on electrical generation was levied in England in 1880.

XI

The Field Be with You

Faraday introduced the concept of field, which he defined as a space surrounding an object, such as an electron or magnet, in which other objects are subject to or affected by a force. The most common example of field is a magnet attracting iron nails. Faraday pictured the space around the magnet or coil as being 'strained' because of the magnetic force. The field concept emerged painfully over many years in many writings, and historians enjoy arguing about how and when it all came out. Faraday noted in 1832, 'When a magnet acts upon a distant magnet or piece of iron, the influencing cause … proceeds gradually from magnetic bodies and requires time for its transmission.' Thus the concept is that a 'disturbance'—for example a magnetic field—can travel through space and notify a grain of iron powder not only that it is there but also that it can exert a force.

Magnetic lines of force are revealed in an old experiment students do in school, in which they sprinkle iron powder on a sheet of paper placed over a magnet. The student gives the paper a tap to break the surface friction, and the iron powder clusters in a definite pattern of lines connecting the poles of the magnet. Faraday thought these lines were real manifestations of his field concept, and he provided detailed, if ambiguous, descriptions of this alternative to action-at-a-distance. Action-at-a-distance refers to the ability of one object to affect other objects without actually contacting them, and Faraday introduced the concept of the field as an explanation of how this can occur. The field concept was altered, used, and ultimately perfected by Maxwell.

XII

At the Speed of Light: James Clerk Maxwell

Although Faraday the inventor changed the world, his science could not stand by itself and would have dead-ended if it were not for the synthesis performed by Scottish physicist James Clerk (pronounced 'klark') Maxwell. Maxwell played Kepler to Faraday's Brahe. Faraday's magnetic lines of force acted as a stepping-stone to the field concept. His extraordinary conclusion in 1832, that electromagnetic actions are not transmitted instantaneously but instead require a well-defined time, played a key role in Maxwell's great discovery.

Maxwell gave full credit to Faraday, even admiring his mathematical illiteracy because it forced him to express his ideas in 'natural untechnical language.' Maxwell claimed that his primary motivation was to translate Faraday's view of electricity and magnetism to mathematical form. But the treatise that evolved went far beyond Faraday’s discoveries.

In the years from1860 to1868, Maxwell's papers—models of dense, difficult, complicated mathematics—emerged as the crowning glory of the electrical period of science that had begun in dim history with amber and lodestones. Maxwell not only set Faraday to mathematical music but, in so doing, also established the existence of electromagnetic waves moving through space at some finite velocity, as Faraday had predicted. This was an important point, as many of Faraday and Maxwell's contemporaries thought forces were transmitted instantaneously. Maxwell specified how Faraday's field would work. Faraday had found through experimentation that changing a magnetic field generates an electric field. Maxwell, struggling for symmetry and consistency in his equations, postulated that a changing electric field would generate a magnetic field. This produced, in the mathematical stuff, a surging back and forth of electric and magnetic fields, which, in Maxwell's notebooks, took off through space, speeding away from their sources at a velocity that depended on all kinds of electrical and magnetic quantities.

But there was a surprise. Not predicted by Faraday, and essential to Maxwell's major discovery, was the actual velocity of these electric magnetic waves. Maxwell pored over his equations, and after he plugged in the proper experimental numbers, out came 3 x 10⁸ meters per second. This number just happens also to be the speed of light, which had been measured for the first time a few years earlier. Maxwell knew that few real coincidences occur in science, and he therefore concluded that light is but one example of an electromagnetic wave. Electricity, it follows, need not be confined to wires but can disseminate through space as light does. 'We can scarcely avoid the inference,' wrote Maxwell, 'that light consists in the transverse undulations of the same medium which is the cause of electric and magnetic phenomena.' Maxwell opened the possibility, which Heinrich Hertz later seized, of verifying his theory by experimentally generating electromagnetic waves. It was left to others, including Guglielmo Marconi and a host of more modern inventors, to develop the second 'wave' of electromagnetic technology: radio, radar, television, microwave, and laser communications.

Here is the way it works. Consider an electron at rest. Because of its electric charge, an electric field exists everywhere in space, stronger near the electron, growing weaker with distance. The electric field 'points' toward the electron. To detect the field, place a positive charge anywhere, and it will feel a force pointing toward the negative charge of the electron. Now, force the electron to accelerate up a wire and two things happen. The electric field changes, not instantly, but as soon as the information arrives at the point in space where we are measuring it. A moving charge is a current, so a magnetic field is created.

Now apply forces on the electron so that it surges up and down the wire at a regular cycle. The resulting change in electric and magnetic fields propagates away from the wire with the velocity of light. This is an electromagnetic wave. The wire is called an antenna, and the force driving the electron is a radio frequency signal. The signal, with whatever message is contained in it, propagates away from the antenna at the speed of light. When it reaches another antenna, it will find plenty of electrons, which it will, in turn, force to jiggle up and down, creating an oscillating current that can be detected and converted to video and audio information.

Despite his monumental contribution, Maxwell was anything but an overnight sensation. It took more than a decade of further experimentation for his ideas to become accepted, a triumph that Maxwell, alas, did not live to experience.

XIII

Hertz to the Rescue

The true hero of the unification of electromagnetism and light is Heinrich Hertz, who, in a series of experiments from 1873 to 1888, confirmed all the predictions of Maxwell's theory.

Waves have a wavelength, which is the distance between crests. The crests of water waves in the ocean typically may be 6 to 9 m apart, while sound wavelengths range in centimeters. Electromagnetism also comes in waves. The difference between various electromagnetic waves—infrared, microwaves, X rays, radio waves—is simply a matter of their wavelengths. Visible light—blue, green, orange, red—is in the middle of the electromagnetic spectrum. Radio waves and microwaves have longer wavelengths. Ultraviolet, X rays, and gamma rays have shorter wavelengths.

Using a high-voltage coil and a detection device, Hertz found a way to generate electromagnetic waves and measure their speed. He showed that these waves had the same reflection, refraction, and polarization properties as visible light waves and that these waves could be focused. It turned out that Maxwell was right. Hertz, in subjecting Maxwell's theory to rigorous experiment, clarified and simplified it into a system of four equations.

After Hertz, Maxwell's ideas became generally accepted, and the old problem of action-at-a-distance was put to rest. Forces in the form of fields propagated through space with a finite velocity, the speed of light. Maxwell believed that he needed a medium to support his electric and magnetic fields, so he adapted the popular notion of an all-pervading ether, in which the electric and magnetic fields vibrated. This part of Maxwell’s theory would be discarded when the existence of the ether was disproved by experimentation in the late 1800s.

The Faraday-Maxwell-Hertz triumph spelled another success for reductionism (the idea that complex forces can be explained as aspects of a single force). No longer did universities have to hire a professor of electricity, a professor of magnetism, and a professor of light or optics. Since these subjects are all unified, only one staff position is now needed (more money for the football team). A vast set of phenomena was thus encompassed, including things created by science and technology and things belonging to the natural world—things like motors and generators, transformers, and an entire electrical power industry, and things like sunlight and starlight; radio, radar and microwaves; and infrared and ultraviolet light and X rays and gamma rays.

Maxwell’s four simple equations are famous in the world of physics, and physics and engineering students the world over wear T-shirts sporting them. For our purposes, it is not important to list them or explain their workings. The point is that they symbolize the scientific summons, “Let there be light!”

Maxwell's original equations were very long and bulky. Hertz rewrote them into much simpler versions that were easier to work with and understand. Hertz was a rare example of a scientist who was more than the usual experimenter with only a working grasp of theory. He was exceptional in both areas. Like Faraday, he was aware of, but uninterested in, the immense practical importance of his work. Hertz's theoretical work consisted largely of reducing and popularizing Maxwell’s theory, an effort that made great strides in physicists’ continuing quest to unify all the forces of nature.

As the 19th century drew to a close, the powerful combination of Newton’s laws of motion, Newton’s law of gravity, and Maxwell’s equations provided the tools needed to explain seemingly all scientific phenomenon. Gravitational force explained the solar system and the motion of objects, matter was somehow made of electrically charged objects, and Newton’s law of motion enabled calculations that showed how these charged objects could attract to make atoms. The chemists had already proven the existence of atoms; only their detailed structure remained to be discovered. In 1897 British physicist Sir Joseph J. Thomson discovered the electron, further clarifying the basic structure of matter. The laws of thermodynamics, generated in part by Faraday’s discoveries, also helped round out the scientific picture by giving direction to chemical reactions and physical processes, and ushered in the beginning of the Industrial Revolution.

Optimism about the so-called end of science, the belief that all scientific mysteries could be understood, began to fade around 1900. With the end of the millennium, it became increasingly clear that some of the puzzles of physics could not be solved by the Newton/Maxwell set of intellectual armaments, however powerful they might be. The challenge inherent in this realization heralded a revolution, one that would prove to make the 20th century the most scientifically productive era in the history of humankind.

About the author: Physicist, professor, and author Leon M. Lederman is director emeritus of the Fermi National Accelerator Laboratory in Batavia, Illinois. Lederman won the Nobel Prize in physics in 1988 for his work with elementary particles. He has taught at Columbia University in New York City, the University of Chicago, and the Illinois Institute of Technology in Chicago.

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