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Happy Birthday, Ben Franklin
JANUARY 17, 2006, was the three hundredth anniversary of Benjamin Franklin's birth. As you no doubt know, Benjamin Franklin was one of our country's Founding Fathers, which means he was a troublemaker. (If you don't believe us, ask King George.) Franklin was also a writer, a philosopher, a statesman, a printer, a scientist, and an inventor. As you might guess, we're most interested in these last two occupations.
Franklin made scientific discoveries in a wide range of fields. While he was postmaster general, he invented an odometer and used it to measure the length of postal routes. He invented the Franklin stove, a urinary catheter, bifocals, and the lightning rod.
Though we respect Franklin's industry and inventiveness, Pat has a bone to pick with him. Her complaint relates to the area in which Franklin made some of his greatest discoveries: electricity.
But before we get to Pat's complaints, we'll tell you a little about electricity. Usually, at this point in our column, we connect our topic to science fiction, citing this story or that novel. But when it comes to electricity, Pat maintains that the connection is actually to fantasy. Understanding electricity requires accepting the existence of worlds that you can't see or experience directly. When you start poking around, trying to figure out what's going on, you find out things are much weirder than you ever figured. You don't exactly open a door in the back of a wardrobe and walk through into another world, but close enough.
Just as so many fantasy novels begin in our familiar world, we will begin with something familiar: the spark of static electricity that jumps from your finger to a doorknob after you walk across a wool carpet. That's static electricity or electrostatics.
In Franklin's time, scientists in Europe had been studying electrostatics for hundreds of years. They knew that if you rubbed wool on the fossilized tree sap known as amber, then the amber would attract little pieces of paper. In 1600, William Gilbert coined the name for the science of electricity from the Greek name for amber: elektron.
In 1660, Otto Von Guericke experimented with a spinning sulfur ball about the size of a child's head. Rubbed with his hand, the ball made sparks and attracted bits of leaves, gold dust, and snips of paper. A woodcut of the period depicts a more elaborate experiment in which a child suspended on silk ropes rubbed a spinning ball of sulfur with one hand while attracting bits of paper with the other.
In 1746, Pieter van Musschenbroek experimented with collecting the electric charge produced by an electrostatic machine in a device later called the Leyden jar. (He got a nasty shock in the process.)
The Leyden jar not only revolutionized the study of electrostatics, it also became a popular sensation. In the 1750s, experimenters all over Europe demonstrated electricity with Leyden jars, often sending the charge through chains of people holding hands. In a demonstration for King Louis XV, French clergyman and physicist Jean-Antoine Nollet sent a current through a chain of 180 Royal Guards, making all the soldiers jump simultaneously.
Through experimenting, people made discoveries about how electricity behaved. But they had had no simple theory to explain the results of their experiments. What was it about rubbed amber that made it exert an invisible attractive force on the distant pieces of paper? What exactly was stored in the Leyden jar? No one knew. Based on their experiments, European experimenters surmised that there were two kinds of electricity: vitreous, which you got by rubbing glass with silk, and resinous, which you got by rubbing amber or resin with wool.
That's where Ben Franklin came into the picture.
Ben Franklin read about the European experiments and repeated them. He explained the results with a single form of electricity, hypothesizing that there was an "electric fluid." There could be too much of this fluid (a condition he called plus) or too little (a condition he called minus). This fluid could move from regions of excess to regions that were depleted.
Of course an object could also have neither excess nor deficiency and be neutral or have zero electric charge. Franklin also noted that an object in the plus condition attracted an object in the minus condition. Plusses repelled each other and minuses repelled each other. (From this we get the saying that "Like charges repel and opposite charges attract.")
Franklin designated vitreous electricity (which you got by rubbing glass with cloth) as "plus." It was an arbitrary choice. He could equally well have called resinous electricity "plus." We'll get back to that (and explain why Pat regards Franklin's choice as troublesome), in a bit. But before we do, we want you to do a little experimenting.
THE MOST AMAZING ELECTROSTATIC DIAGNOSTIC TOOL EVER
Not long after the invention of the Leyden jar, Jean Nollet (yes, the same one who made those soldiers jump) invented the electroscope, a device for detecting electric charge. Today you can build your own electroscope with a roll of Scotch® brand Magic tape. (Yes, it has to be Scotch® brand. Don't substitute any other kind!)
Here's what you do: Take two pieces of tape, each one about as long as your hand is wide. Stick the sticky side of one tape onto the non-sticky side of the other. Now pull the two tapes apart quickly.
As the tapes separate, they will grab your hand. When you free them from your hand, they will attract and stick to each other. If you and a friend do this at the same time, one of your tapes will attract one of your friend's tapes and repel the other.
Pulling apart two pieces of tape causes one to become positively charged and the other to be negatively charged. To find which is which we can return to Ben Franklin (who didn't have any Scotch® brand tape, but did have something to say about charge). Franklin defined negative charge as the charge on the amber when it was rubbed with wool. If you don't have a large lump of amber around the house, you can substitute your hair for the wool and a rubber or plastic comb for the amber. Run the comb through your hair and it will pick up a charge that Ben Franklin would define as negative, the same as amber rubbed with wool.
Bring the negatively charged comb near one of the tapes, and then the other. The tape that the comb repels is negatively charged. The tape that the comb attracts is positively charged. Now you can bring your tapes near objects with unknown charges and determine whether those objects are positively charged, negatively charged, or neutral. (An object that is uncharged or neutral will attract both tapes.)
While you're experimenting, bring your tapes near the front of an operating television—an old television with a picture tube, that is, not a plasma screen or LCD display. Notice that the screen is positively charged. That's why TV screens get covered with dust. The positively charged screen attracts the neutral dust particles.
FANTASY? OR SCIENCE FICTION?
Let's take a minute and talk about what is going on when you tear those pieces of tape apart.
When you pulled your tape sandwich apart, you also ripped apart some atoms!
Here's where Pat says we get into the realm of the fantastic. Like everything else in your house, the tape is made of particles too tiny for you to see. These particles are called atoms. Atoms are made of even tinier particles called electrons, protons, and neutrons. Both electrons and protons are electrically charged particles. Electrons are negatively charged and protons are positively charged.
When you pull the tape apart, you are pulling some electrons away from their protons. One piece of tape ends up with more electrons—it's negatively charged. The other one ends up with more protons and it's positively charged.
That's the story that the physicists tell—and it's a pretty good story. It explains a lot of things.
Positive charges and negative charges like to stick together. (Paul says physicists don't know yet why they attract each other. They just do.) The two pieces of tape are attracted to each other because one is positively charged and the other is negatively charged.
When you run a comb through your hair, the comb ends up with excess electrons and becomes negatively charged. Since negative charges push away other negative charges, the comb pushes the negatively charged tape away and attracts the positively charged one.
THE MYTHICAL CURRENT
That's all well and good. Pat would have no complaints with Benjamin Franklin if electrical experimentation had stopped with electrostatics. But it didn't. In 1799, Alessandro Volta created the first electrical battery, known as the Voltaic cell. In 1821, Michael Faraday began experimenting with devices that led to the development of the electric motor and the electric generator. One thing led to another, and we ended up where we are today—in a world largely powered by electricity.
Consider, if you will, one situation in which electricity is used. Suppose you turn on the headlights in your car. Your car's battery has a positive terminal and a negative terminal. A wire runs from the positive terminal, through the headlights, through a switch, to the negative terminal. When you turn on the headlights, the switch closes, letting an electric current flow through the wire and through the headlights, causing the filament in the headlights to glow.
So far, that description matches Franklin's way of talking about electricity. He said that the electrical fluid moved from one place to another. He would say that the current flows from the positive terminal (the place of excess electric fluid, to his way of thinking) to the negative terminal (the place deficient in electric fluid).
According to the physicists, that's not what's going on at all. They say that the electrons are moving. When you turn on the headlights, you throw a switch and complete an electric circuit. That circuit is a path along which negatively charged electrons can move.
Electrons move from the negative terminal through the headlights to the positive terminal. Though the lights come on as soon as you throw the switch, an electron will take several hours to make the journey from the negative terminal to the positive terminal. (But that's a story, which we'll save for another day.)
So here's the problem. Electricians talk about electricity as Franklin did. They treat electric current as if it goes from the positive terminal of the battery, through the circuit, to the negative terminal. That's how they talk about it. They give this mythical flow of positive charges a name: the conventional current.
Even after all these years at the Exploratorium, there are days when Pat is not so sure she believes in atoms, electrons, protons, and other particles too small to see. (This is a situation that will be corrected or exacerbated over the next year. Pat is currently working on a project dealing with nanotechnology, which is all too small to see. More on that next column.) But even on days when Pat accepts atoms without blinking, even on days when she can accept the flow of electrons through the wire, she has a problem with the flow of positives that engineers talk about.
There is no flow of positives. That positive flow is the legacy of Ben Franklin and his electric fluid. The original model for understanding electricity—which was very useful when Franklin came up with it—lives on to make trouble.
Of course, that's the way science works. Any model we have for the universe—whether it's a model for light or a model for electricity or whatever—breaks down at some point. The model is not the thing. The model is a human invention—a fantasy—intended to explain the thing. It's a way of understanding the invisible, the intangible.
The trick in science is picking the simplest model that gives the right answer. And Franklin did that as well as he could at the time. He realized that the model involving the two kinds of electricity postulated by others could be simplified, and he did so. And in doing so, he left us with a mythical current that flows in the direction that electrons do not.
ABOUT THOSE LIGHTNING RODS
We can't leave a discussion of Ben Franklin and electricity without mentioning the story that every schoolchild knows: Ben Franklin flew a kite in a thunderstorm. Why? To generate sparks so that he could compare the behavior of those sparks to the charges generated in his down-to-earth experiments. As a result of this experimentation, he proposed that lightning was nature's own electrostatics experiment.1
Having made the connection between electrostatics experiments in the laboratory and those in nature, Franklin found a practical application for his new ideas and invented the lightning rod. A lightning rod is a metal rod sticking up from the rooftop of a building and connected by a good conductor (a heavy copper wire) into the earth.
A full explanation of how a lightning rod works is a little tricky. Paul can demonstrate how it works with a Van de Graaff generator, the static electricity device near to the hearts of science demonstrators everywhere. Suppose you had set up a metal object near a Van de Graaff generator and had sparks jumping happily from the generator to the metal object. Suppose you used an insulated handle to hold a pointed metal rod connected to the earth (referred to by electricians as the "ground") and bring the point near the Van De Graaff generator. The sparks would stop.
Why? Because when a pointed object is electrically charged, electric forces are concentrated near the point. These forces rip apart air molecules, ionizing them. The space around the point is filled with charged particles, which discharge the generator faster than charge can build up.
The same thing happens with a lightning rod. The pointed rod discharges the local electrostatic charges and helps prevent a lightning strike.
Franklin's discovery that lightning is an electrical discharge had an enormous social impact. With Franklin's invention, people could protect themselves from lightning, a wild force of nature. At the time, this accomplishment was regarded by many as being just as important as his role in the American Revolution. As Anne-Robert Jacques Turgot wrote of him in 1776: "Eripuit caelo fulmen, sceptrumque tyrannis" ("He snatched lightning from the sky and the scepter from tyrants").
Since Franklin's time, scientists have continued studying lightning. Yet they still do not have a model that explains it fully. They know that in most cumulonimbus clouds a region of negative charge forms near the bottom of the cloud while a region of positive charge forms near the top. But they don't know the mechanism by which the charge is separated inside the cloud, or why falling raindrops leave some of their charge at the bottom of the cloud as they continue to fall to the ground.
To measure the charge distribution in the clouds, adventurous scientists fly gliders into these thunderstorms with an electronic version of your tape electroscope. Did you ever wonder what you can do with a bachelor's degree in physics? At the National Center for Atmospheric Research, Paul once saw a job listing looking for someone with a bachelor's degree in physics, a glider pilot's license , and parachuting experience. The job was flying gliders into thunderstorms. Why did the applicant need parachute experience? Because the thunderstorms sometimes ripped the wings off the gliders!
You now know better than to fly a kite in a thunderstorm, but there are plenty of dangerously intriguing scientific puzzles yet to be solved for those who would like to follow in Ben Franklin's footsteps.
1 Franklin was lucky. When Georg Richmann repeated this experiment in St. Petersburg lightning struck the kite, came down the string and killed the experimenter. If you learn one thing from this story it is this: don't fly a kite in a thunderstorm!
The Exploratorium is San Francisco's museum of science, art, and human perception—where science and science fiction meet. Pat Murphy and Paul Doherty both work there. We'd like to thank the Exploratorium for the use of figures 2 and 3, created by David Barker. We'd like to thank all the members of the Exploratorium's Relativity team (Thomas Humphrey, Ron Hipschman, Dave Barker, Noah Wittman, Judith Brand, Sarah Reiwitch, Jenny Villagran, and Ruth Brown) for their inspiration and their contributions to the ideas in this column. To learn more about Pat Murphy's science fiction writing, visit her web site at www.brazenhussies.net/murphy. For more on Paul Doherty's work and his latest adventures, visit www.exo.net/~pauld.
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