Electric Universe

by David Bodanis

  MR. AMP, MR. VOLT, AND MR. WATT

  The world is made of electric charges, and our technologies operate through electric charges, and even our brains are powered by electric charges. Yet how to measure the flow of electricity? Three historical characters have had their names immortalized in the familiar units amps, volts, and watts, which describe—and summarize—what’s happening inside all the electric devices we use.

  The first was the French mathematics professor André-Marie Ampère, who in 1820 studied the way that rushing electrical currents created magnetic pulls. It was one of the only satisfying moments in his life—on his tombstone he had the phrase Tandem felix (Happy at last) inscribed—but in 1881, long after his death, the word amp was taken to measure the flow of charged particles. In ordinary household circuits, it’s simply a count of how many electrons are passing through a given point in a wire each second. When 6 quintillion (6,000,000,000,000,000,000) electrons pass in a second, then we say that one amp of current is flowing; when 12 quintillion electrons travel through the wire, we call it two amps of current. An electric bulb lights up when a single amp’s worth of electrons—6 quintillion individual electrons—are passing along each second. Switch on a car’s ignition, and 50 amps’ worth of electrons—300 quintillion electrons—pour into the spark plugs each second.

  The second character was the prickly Italian Alessandro Volta. He created the batteries that allowed other researchers to investigate the force that can “push” all those electrons along, though, as we’ve seen, he had little understanding of why those batteries worked.

  Michael Faraday’s explorations helped make sense of that pushing force, and the key concept for measuring it came from Macquorn Rankine, a friend of both William Thomson and Faraday. Rankine had spent years designing railroads for the rolling hills of Scotland, and he used that experience to help come up with the notion of what he termed “potential energy.” A train high on a hill might not be moving fast, but it has a lot of this potential energy, for once it starts down the hill it’ll pick up speed thunderingly fast.

  Rankine and Thomson used that vision to try to look into the force fields they imagined stretching out from batteries and other electric sources. In their mind’s eye, there were invisible “hills” where the field was strong, and “valleys” where it was weak. Put a charged particle where the field is strong and it’ll hurtle away, like happily startled train passengers who see their locomotive leading them fast down to a waiting glen. A “volt,” Rankine and Thomson realized, should simply measure how steep—and so how strong—this pushing force is in the topography of one of Faraday’s invisible fields. An amp measures how densely a current of electrons is flowing; a volt is the “downhill” pushing force that creates this flow.

  The last of the three characters was James Watt, yet another ingenious Scot, and the patron saint of electric bills. Although he hadn’t quite invented the steam engine, he had improved it greatly, and decided that his major achievement would be to persuade tight-fisted mine owners to actually buy this new machine.

  What he needed, he realized, was a risk-free offer, showing that their savings from the new machine would more than offset the cost of buying it. But for that he needed to measure the savings, which meant he had to find a catchy way of summarizing what the horses dragging the carts and driving the pumps in the mine were doing.

  He found that a horse could tug a 500-pound weight quite steadily, and came up with the idea of “horsepower”: the rate at which an average horse could keep working through a long day. If he could offer a mine owner a steam engine that cost less than a horse and its feed, yet would operate at more than the rate of one horsepower, then, he realized, he had a chance of making a sale. With the spread of the metric system, his original term was replaced, most justly, by the term watt. Horses are strong, and the power of a single watt was defined as a small fraction—just about ⁄⁄¶(infinity)ºth—of a single horsepower. Today’s DJs don’t use horses tugging on pulleys to spin their recordings or pump their speakers, but when they say they’re going to power up a 750-watt sound system, they’re basically saying that their equipment needs as much power as a single tugging horse could provide. A watt simply measures the power that the pushing volts and scurrying amps will provide.

  In the time of Mr. Watt’s descendant Robert Watson Watt, Britain’s radar defense depended on being able to compute how this accurately measured power would travel through the air. The sky is loaded with electric charges, yet they’re so tightly bound up with one another in air molecules that even a powerful radio wave, shooting upward, won’t be able to drag them apart. Only the far stronger force fields emanating from huge storm clouds can do that, to begin the crackling of a lightning bolt. But would the weak beam of a radio broadcast be enough to tug loose at least some of the electrons in a soaring metal airplane?

  That’s what Arnold Wilkins had to compute in Slough in 1935, and he used Mr. Watt’s ideas—and the clear definition of watts that the original definition of horsepower provided—to work out that there would, in fact, be just enough power transmitted outward to make those electrons transmit a detectable answering signal back toward the ground. Power measured in watts would fly up, a field with a pushing force measured in volts would accordingly appear around the plane, and a flow of electrons measured in amps would start up inside the airplane’s metal wing—strongly enough, Wilkins computed, to create the answering signal on which Britain’s radar defense could depend.

  NOTES

  Conveniently vain…Alessandro Volta

  Volta’s discovery didn’t come out of the blue, for he’d been working on devices to transfer or measure static electricity for over a quarter-century. But when Luigi Galvani—a mere anatomist—found what seemed to be a source of moving electricity, the semi-aristocratic Volta was appalled, and his research went into overdrive.

  Volta is surprisingly restrained in the key paper where he announced his battery to the Royal Society, but the true stories of berating competitors, and especially the unfortunate Galvani (who got the explanation of the bimetal effect almost entirely wrong) is well described in Marcello Pera, The Ambiguous Frog: The Galvani-Volta Controversy on Animal Electricity (Princeton: 1992). The Fara and Heilbron texts in the reading guide for this chapter give wider background.

  Tingling sensation…across his tongue

  The atoms of the two metals had different configurations of electric force on their surface, but that alone would not propel a current across Volta’s tongue. Saliva, however, is largely salt water, which made it reactive enough to combine with the zinc, so that microscopic fragments from the zinc floated away into Volta’s mouth. This meant that instead of balanced amounts of electric charge remaining on each disc, a buildup of raw electrons began to appear.

  With those extra electrons in position on the zinc, the potential “push” between the two different metals could get to work. Sulfuric acid is even better at reacting with metal to feed electrons into one of the waiting terminals, hence its use in later batteries.

  Note the crucial role of the liquid. The pressure or “oomph” between the two metals—the voltage—depends on the nature of the metals involved. That’s why batteries have standard voltages such as 1.5 volts, and why Volta was safe from electrocution: his coin-shaped discs couldn’t generate much voltage. But the energy to tear apart the zinc atoms, and supply the electrons to keep on taking that ride from one metal to the other? That comes from the sizzling liquid, lapping away around the metals—and that’s why Volta would continue to feel the tingling sensation, so long as he kept the coins in his mouth.

  Curiously, our civilization isn’t the first to use batteries. In the 1930s, a strange, rusted, urn-like device was found in Iraq that was almost certainly a battery. It dated from the third century A.D. and had an iron rod at its center, neatly separated from a copper sheath. When twentieth-century researchers rebuilt it and put vinegar in to dissolve extra charges on the copper, they found it produced a stead
y one-volt output. But whether it was used simply to electroplate jewelry, or in terrifying, spark-producing priestly rituals, archaeologists cannot tell.

  The world’s first steadily operating “battery”

  The name had first been used for groups of metal-coated glass containers, called Leyden jars, which could store accumulations of static electric charges. (A repeated grouping of identical objects is often labeled a “battery,” as with a battery of artillery.) But Leyden jars could release only a single sudden shock. Volta’s construction was superior because it produced a steady current.

  A single pair of metals provides only a small amount of the pushing force that Volta noticed, but connecting another pair of metals to the first doubles the effect; connecting a third pair triples it. The diagram Volta attached to his Royal Society paper showed dozens of metal pairs connected by wires; a modern auto battery has similar layers. Giuliano Pancaldi’s Volta: Science and Culture in the Age of Enlightenment (Princeton, NJ: Princeton University Press, 2003), 246–48, shows the gradual domination of the word battery over competing phrases such as trough or cell (the latter of which lingers on in our term fuel cell).

  A primitive mobile phone…in 1879

  The creator was David Hughes, an American engineer living in London, and he used a trolley to carry the “mobile” phone, which created audible clicks when it picked up sparks that were generated up to five hundred yards away. There’s a copy of the original at London’s Science Museum.

  That filled empty space

  You can watch the Big Bang on television now. Only a very small portion of the radiation seething through space in the universe’s earliest moments was used up in creating the charged particles from which we’re built. Most of it kept flying loose, and since it’s simply electromagnetic radiation, similar to that of television broadcasts, we pick up some of it whenever a TV set is tuned between stations—creating perhaps 1 to 5 percent of the snow-like static that fills the screen.

  Judicious financial involvement…with Congress

  Morse secretly hired the chairman of the House Commerce Committee, Francis O. J. Smith of Maine, promising him substantial profits if the committee were given the needed report. Shortly after the committee gave its favorable report, Smith retired and took up a position as Morse’s partner. He got rich from Morse’s patents, and the government funds he’d just voted for, then left Morse’s employ and tried to get richer by blackmailing Morse.

  In 1875, his…invention came together

  Many other inventors were active in the field, not least the unfortunate Elisha Gray, who filed for a patent just hours after Bell. Gray wasn’t too perturbed, though, for he suspected that the speaking machine was just a distraction from his main research on telegraphs. Even Bell was confused, at least for a while, and explained to investors that what he’d created was merely a telegraphic machine that didn’t need experts to translate its signals.

  But that’s basically how a telephone works

  The ringing we hear when we phone someone doesn’t come from the telephone of the person we’re calling. Rather, it’s a signal being sent to us from a central switching station to give us the impression that we’re hearing that phone. It’s a trick that dates from the early days of central exchanges. (If the two signals fall out of sync, the illusion vanishes; this is when the person at the other end of the phone hears a ring and answers before we’ve heard a sound.)

  Edison thought about it and saw there

  Edison was an unscrupulous man but a deft engineer. The way he modified the battery’s current was by putting an obstacle in its way: a tiny box, filled with powdery granules of carbon.

  When someone speaks loudly into such a telephone, those carbon granules are blown tight against one another. But when carbon is jammed together like this, electric currents travel across it more easily—think of how much easier it is to advance across steppingstones that are suddenly brought closer together. Only as the human voice goes back to being softer do the carbon granules stop being squeezed so tight. The carbon in the small box is powdery and loose again, and so less of the battery’s current makes it across. His invention continued to be used for almost a century.

  “I had no time to do anything more about it”

  And if he hadn’t been so busy, he quite possibly would have invented the television. The black dot that Edison saw came from streams of electrons boiling out from his lightbulb’s filament. He’d already found he could partly control the beam by putting a bit of tinfoil on the outside of the glass, and if he’d gone ahead and put magnets beside the bulb, he would have found the electrons being tugged from side to side by the magnet’s field; this was the essence of J. J. Thomson’s subsequent experiment. Even with his limited measuring techniques, Edison would have noticed the black dot appearing in different places; had he tried different coatings on the glass, he would have seen different colors appear and glow.

  That’s almost exactly how traditional cathode-ray television sets or computer monitors work. A beam of electrons is shot from the back of a tube that’s basically a big lightbulb; when it hits sensitive molecules coated to the inside of the glass front of the tube, the energy the electrons carry makes those molecules glow. To steer the electrons and create a moving picture rather than just a single heap of dots, magnets on either side of the tube pull the electron beam from side to side, neatly matching signals that the broadcaster is sending out.

  The sparks caused by static electricity

  Static electricity powered the atomic bombs over Japan.

  In most atoms, the powerful electric charges inside a nucleus can’t push away from one another. That’s because what’s termed the strong nuclear force acts like a powerful glue, holding them together. But the nuclear force is only about one hundred times as strong as the force of electricity. As an atom gets close to having one hundred protons in its nucleus, that glue is easily dissolved. Uranium and plutonium atoms are about that size, and so relatively easy to split apart.

  In the bomb exploded over Hiroshima in 1945, charges in the uranium nuclei that had been held together for billions of years were abruptly released. For a very brief moment, there was nothing to hold back the electrostatic repulsion between the protons, and an entire city was flattened as they flew apart.

  An invisible force that spread from a moving magnet

  The final details are modern, but the vision of invisible hovering forces is old. William Gilbert, the court physician to Queen Elizabeth I, wrote that the reason rubbed amber could attract feathers was that a “humour” was being removed from the amber, which led to a powerful “effluvium” floating around it. The words seem odd, but substitute the terms charge and field, and he would then be saying that charge is removed from amber when it’s rubbed, and the result is a changed field around it—which we would take as a rather perspicacious suggestion. For Faraday’s own changing views of what constituted a field, see the guide to further reading for chapter 4.

  His excited theories…were politely set aside

  Or, sometimes, less than politely. The Athenaeum magazine wrote that Faraday should go back and study his school mathematics before venturing into the deep seas of modern physics; the immensely supercilious Astronomer Royal, Sir George Biddell Airy, remarked that “I can hardly imagine anyone who practically and numerically knows [modern electrical theory to accept] anything so vague and varying as lines of force.”

  Airy made a habit of putting down his social inferiors. Aside from being a footnote in the history books for missing Faraday’s work, he’s also a footnote for missing the discovery of Neptune—once again, a scientist from the wrong background presented him with evidence of its predicted orbit, which he refused to examine properly.

  It would also writhe sideways

  In the language of electrical engineers, the different layers of the cable and the saltwater built up a series of “capacitors”—that is, there were two separated conducting surfaces, which both tried to pull some of the current
and charge. That was what produced the distortion of the signal, for when so much of the loaded current was diverted into the cable’s iron skin, only a weak and spread-out signal could get through the main copper core.

  Only very gentle battery pressure could be applied

  By the end of the project, Thomson was powering telegraph messages across the Atlantic using only a thimble and a single drop of sulfuric acid. He would lower two different slivers of metal into the thimble, let the acid react with one of them to create a buildup of electrons (just as Volta’s saliva had reacted with the coins on his tongue), and then connect that to the giant undersea cable. The sliver of metal was tiny, but electrons are tinier still; many billions of extra electrons quickly accumulated on the sizzling metal. From that skimpy “pile” of charged electrons, a force field powerful enough to cross the ocean streaked forward, jostling waiting electrons in a telegraph receiver two thousand miles away.

  Just a force field waiting at the sockets

  The idea of fields is fundamental, but the terminology of “voltage”—to describe the force or “oomph” with which the electrons in a current are pushed—is immensely convenient. The appendix on Mr. Volt touches on this, but to understand the link in some more detail, consider again what happens when you stretch your fingertip close to a metal railing on a dry day after you’ve been unwisely scuffing your feet. The air between your finger and the metal is ordinarily an insulator, for there are hardly any free electrons available to carry a current in ordinary air. If you keep your finger several inches away from the metal you’re not going to get a shock, but if you bring your finger closer to the metal, you’re doing work, from all that effort you’ve spent pushing the charges on your finger closer to the gleaming metal danger. The field is more intense around your finger now, for it’s concentrated in the narrower gap between you and the close metal.