11.3. Electricity: Poor Richard#
There were various attempts to study both electric and magnetic forces, although the idea that there might be a connection between them had not occurred to many people, it was clear to those who came after Newton that some sort of Action at a Distance seemed to be going on. In 1747, Ben Franklin (1706-1790) took time out of his busy social life in London and Paris to experiment and set the course for our understanding of electricity. This man traversed the Atlantic Ocean eight times in his life and his wit and romantic appeal was familiar in Britain and especially France. While he came to it naturally, the Enlightenment fever which by that time had overtaken Paris certainly influenced both Franklin’s politics of individualism and self-determination, and his innate inquisitiveness to try to understand the “how’s” of the natural world. We all know him in scientific lore for a foolhardy experiment in the rain. But he studied many things and it was for electricity where he had his most significant impact in physics.
Franklin’s electrical experiments began in 1747 with a glass rod and silk cloth, and after sufficient experimentation he was inspired enough to write a book of his numerous experiments in electricity four years later. It was here that he guessed that lightning was a manifestation of electricity, and he proposed a method to prove it. While not necessarily due entirely to Franklin, by the time he was done with his scientific work it was generally acknowledged that:
Things everyone knows about electric charges Electrical Charges exert forces on electrical charges: like charges repel and opposite charges attract. Electrical charges can be isolated from one another.
Please answer Question 2 for points:
11.3.1. Franklin: When Your Tool is a Hammer…#
…then everything is a nail. Isn’t that how the saying goes?
Franklin’s model for electricity held sway for nearly a century: he asserted that electricity was a single fluid, just like everyone thought heat was a fluid. Neither was a bad guess, although both were wrong, but one works with the tools that one has and the mathematics of fluid flow and thermodynamics were being developed at that time and became quite sophisticated by the middle of the 19th century. It’s interesting that while electric and magnetic fields are not fluids, the mathematics used to describe them is very similar to that of fluids. So barking up the wrong tree can sometimes still be a worthwhile experience (unless, of course, you’re an actual dog).
Franklin’s “fluid” fit into the porous volumes of all substances. If two bodies had the same amount of fluid, they were neutral. But the fluid could ooze from one to the other and a body which had an excess of the electrical fluid would repel another similarly “full” body, and attract a one who’s porous material contained less. How this fluid exerted its force was not understood and collectively people threw up their hands and attributed it to the same mysterious Action at a Distance attributed to Newton’s Gravitational Law. And like for gravity, the force was instantly felt across otherwise empty space.
Because of this to and fro flowing, Franklin was led to postulate a conservation law, that the total amount of electricity was conserved and only just moved from one place to another. This imaginative and in pictorial process describes one of the most important principles in Particle Physics: that electrical charge is conserved. Remember our definition of Conserved Quantities?
Net Electrical Charge is Conserved.
Just like our discussion of the conservation of energy and momentum, this conservation law is related to a symmetry, but a complicated quantum mechanical symmetry for which there is no classical description. It is one of the most fundamental laws of Nature.
This movement of charge he called a current, the word we use today, in homage to that fluid-inspiration. Of course, we also speak of current flowing from one terminal to another, which in turn implies a direction for current. From Franklin we also get the language of “positive” and “negative” charges. He thought that the body which had been relieved of its fluid was then “negative” and that the fluid itself was an addition, hence “positive” in a body in which it’s accumulated. Of course, today we understand that “flow” in circuits is the nearly free transport of electrons within a copper matrix, which only loosely binds them to a particular nucleus. Today we describe the flow in Franklin’s terms, from positive to negative, but the more dominant actual charge motion is from the negative terminal to the positive.
Moving electrical charges are an electric current.
Currents Usually, we think of a current as negative electrons flowing in a copper wire. While they are confined to the wire any movement of electric charges is a current. One could have a current of electrons without wires…like a lightening bolt…or positive protons in a partile accelerator beam. Both are currents and if the same number of electrons or protons flow, then the magnitude of the currents would be the same.
Franklin named things and electrical matters were no exception, “I feel a Want of Terms here and doubt much whether I shall be able to make this intelligible.” He was inventing a science. In addition to his terms “positive” and “negative,” we use “plus” and “minus” and the symbols \(+\) and \(-\) and the words “charge” and “battery.”
Please answer Question 3 for points:
The Kite
So did Franklin actually fly that kite? Probably not the way the legend holds, certainly not as the Currier and Ives etching in the figure suggests.
First, it had already been done, so he wasn’t first. And second, he would have died had he done this in an actual lightning storm, as happened to Georg Wilhelm Richmann in St. Petersburg in 1753. No, what Franklin probably did in his demonstration in Philadelphia in 1752 was fly a kite high enough to collect some charge along the wet kite string from a storm cloud and then suggest that lightening’s current was from the same source. And then he surely waddled for cover.
11.3.2. Science Follows Technology#
Franklin’s research and that of others who quickly followed was due to their imaginations, but also to some major technological inventions. One of the first was the improvement of the electroscope as shown here.
Using a nearly evacuated bell jar and thin gold-leaf tabs, small amounts of charge could be deposited and relative amounts determined by the separation of the leaves. This was the primary device for measuring electrical charge throughout the 18th and 19th centuries. But it’s hard to generate measurable amounts of electric charges when all you’ve got is muscle-power, glass rods, and silk. Furthermore, it’s delicate work since it’s hard to keep charges isolated as they will quickly bleed away into even slightly humid air.
This situation was made much easier when in 1745 at the University of Leyden, a glass jar was surrounded by a metal can with another metal can on the inside: a cylindrical, metal-glass-metal sandwich as shown here in the figure.
It was found that this “Leyden jar” could be “charged” by successively adding increments of charge, which would stay on the cans and grow to even dangerous amounts. Today we call such an arrangement a “capacitor,” after the fact that a capacity of charge can be stored on it. The Leyden Jar made it possible for researchers to store and then use charges in their investigations without having to create them over and over. Franklin was “filling” a Leyden jar in his kite experiment (which you can see beside his foot in the figure above in the thunderstorm). But charge still needed to be created by frictional means—either by rubbing two materials together by hand, or by more efficient Wimshurst devices, large circular plates with brushes which could be turned by hand as in this “Wimshurst Machine.”
11.3.3. Eel Be Sorry#
One of the most important devices is derived from a fish story. Well. Not exactly a fish. Eels were imported to Europe from South America and Africa as a delicacy in the mid-1700s, and their strange talent of generating enough voltage to charge a Leyden jar made for endless fascination in 18th century Europe. In 1786, the physician Luigi Galvani (1737-1798) was dining in a Bologna restaurant as a thunderstorm was approaching and he noticed that frogs legs hanging near a iron railing were involuntarily twitching. He drew the conclusion that this was the same effect as that of the eel, and studied it.
The biological effects of this “Galvanism” were ghoulishly investigated him and others by discharging Leyden jars in the bodies of guillotined prisoners and their severed limbs. You can only imagine how startling it must have been to see the dead move and public demonstrations made the phenomenon a popular topic. (This “cutting edge” research is known as one of the inspirations for Mary Shelley’s creation of Frankenstein.) Another Italian, Galvani’s friend Alessandro Volta, noticed that two dissimilar metals in salt water would produce the eel’s effect inorganically and in 1800 Volta constructed a sandwich of copper and iron/zinc disks, separated by cloth soaked in salt solution. This “Voltaic Pile” is shown on the right.
This was the forerunner of the battery which made it possible for natural scientists to create large amounts of charge, which they could store in their Leyden Jars and to further study their motion—currents.