Electricity 101 with Franklin

Lesson 1

Static electricity and the Leyden jar

Rub a balloon on a sweater and hold it near your hair, and you are doing the oldest experiment in electricity. Two thousand years before Franklin, Greeks noticed that rubbed amber picks up feathers and straw. By Franklin's century the parlour version had grown up: glass tubes rubbed with silk crackled, sparked, attracted cork balls and repelled them, and gave experimenters lively shocks. This was static electricity, charge gathered and held in one place, and it was mostly a toy until a bottle turned it into something you could store, carry, and unleash all at once.

The bottle was the Leyden jar, stumbled on in 1745 and 1746 in Germany and in Leyden in Holland. It is a glass jar coated inside and out with metal foil, with a rod through its cork touching the inner coat. Connect the rod to a rubbing machine and the jar charges silently, giving no sign of the violence it holds. Then touch rod and outer coat together with a wire, or, unwisely, with your hands and body, and the whole charge crosses in one snap and a convulsive shock. Professor Musschenbroek, who tried it in Leyden, wrote that he would not take a second shock for the kingdom of France.

Franklin took the jar apart with his mind and found its secret, and it is the seed of this whole course. The glass itself holds the charge apart: as the inside gains, the outside loses, in exact balance. In his words, “whatever quantity of electrical fire is thrown in at top, an equal quantity goes out of the bottom.” Nothing is created; charge is moved and separated, and the shock is the separated charge rushing home through whatever connects the two coats. Every modern capacitor, the tiny charge-storing component inside every phone and radio, is the Leyden jar's direct descendant, still obeying Franklin's balance.

Lesson 2

Franklin's kite and the lightning rod

Franklin's boldest idea was an identification: the spark from a rubbed tube and the lightning bolt are the same fire at different scales. He proposed a test in print before anyone performed it: put a man in a sentry box on a high tower with a pointed iron rod rising twenty or thirty feet into the air, and see whether passing thunderclouds would electrify the rod. Experimenters in France carried out his proposal in May 1752 and drew sparks from a cloud. Weeks later, by the famous account, Franklin flew a silk kite in Philadelphia with a pointed wire on top, a hemp string that would conduct when wet, a dry silk ribbon to insulate his hand, and a key tied where the two met. As a cloud passed, loose fibres of the string stood up and a spark jumped from the key to his knuckle. He charged a Leyden jar from the key and found its fire behaved exactly like machine-made electricity.

Now the safety line, in the plainest words this course owns: DO NOT TRY THIS. Never fly a kite in a storm, never hold a rod or wire up toward thunderclouds, and never try to collect charge from lightning. Lightning kills, and it nearly collected the experimenters themselves. In 1753 the physicist Georg Richmann in St Petersburg was killed at his apparatus while repeating this kind of observation, struck when a discharge came down his rod. Franklin's own margin was luck as much as design: a direct strike on his kite string would very likely have killed him. Historians also handle the kite story with care. Franklin never published a full account of it himself; the detailed story comes from Joseph Priestley fifteen years later, and some details may have grown in the telling. The solid core is the French sentry box success, Franklin's published reasoning, and jars charged from the sky by others.

The practical fruit of the idea matters far more than the kite. If points draw electrical fire off silently, Franklin argued, then fit buildings with “upright rods of iron made sharp as a needle,” joined by a wire down the outside wall into the ground, and the rod will either drain a passing cloud quietly or give a strike a safe road to earth that spares the house. He was right, and the lightning rod became his gift to every church steeple, barn, and ship's mast in the Atlantic world. Church bells rung against storms had killed bell ringers for centuries. A cheap iron rod, reasoned out from parlour sparks, ended that.

Lesson 3

Positive and negative charge

Before Franklin, experimenters spoke loosely of two kinds of electricity, one from rubbed glass and one from rubbed resin, as if nature kept two different electrical fluids. Franklin simplified the whole subject with one stroke. Suppose, he said, there is a single electrical fire, spread through ordinary matter in a natural amount. A body with more than its natural share is electrified one way; a body with less is electrified the opposite way. Rubbing does not create the fire. It pumps it from one body into another, so every gain somewhere is matched by a loss somewhere else, exactly as the Leyden jar showed, top against bottom.

His names for the two states are the ones on every battery in your house. The surplus state he called positive, or plus, and the deficit state negative, or minus. Go back to his jar analysis and you can watch him coin the bookkeeping: at the same moment, the top of the bottle is electrified “positively or plus” and the bottom “negatively or minus, in exact proportion.” Like charges, he showed, repel each other; unlike charges attract; and a charged body attracts a neutral one. With plus, minus, attraction, repulsion, and strict conservation, the parlour sparks became a science with accounts that balance.

One honest footnote shows how conventions outlive theories. Franklin had to guess which state to call plus, and he assigned plus to the charge on rubbed glass. Two centuries later we learned that what actually moves in a metal wire is the electron, which carries the charge Franklin would have called minus, so conventional current is still described as flowing from plus to minus while the electrons drift the other way. Engineers kept Franklin's labels anyway, because the bookkeeping works perfectly either way. When you see the little plus and minus signs moulded into plastic at the ends of a battery compartment, you are reading Benjamin Franklin's handwriting.

Lesson 4

Magnets and magnetic fields

Electricity's older sibling is magnetism, known since antiquity through the lodestone, a natural iron ore that picks up nails and, floated or suspended, swings until it points north and south. Sailors built the compass on that swing centuries before anyone understood it. Every magnet, natural or made, shows the same anatomy: its power concentrates at two ends, its poles, and the rules of the poles are simple enough to learn with two bar magnets on a table. Like poles repel. Unlike poles attract. Bring north to north and the magnets push apart with an invisible spring; bring north to south and they snap together.

Try to cut a magnet's poles apart and nature refuses. Saw a bar magnet in half and you do not get a north piece and a south piece; you get two smaller complete magnets, each with both poles. The poles are not things inside the magnet but ends of a pattern running through the whole bar. That pattern can be made visible, and Silvanus Thompson's Elementary Lessons, the classic textbook this course pairs with Franklin, teaches the picture every physics class still uses. Sprinkle iron filings on paper over a magnet and tap gently: the filings chain themselves into curving lines that stream out of one pole, arc through the air, and dive into the other. Those lines map the magnetic field, the region in which the magnet's influence acts, strong where the lines crowd and weak where they spread.

The grandest magnet in the story is the Earth itself, which behaves as if a colossal bar magnet lay near its axis, which is why compass needles everywhere agree on a direction. William Gilbert argued exactly this in 1600 from experiments with spherical lodestones, and Thompson's lessons carry the same picture forward for school students. Hold the field idea firmly, because the rest of this course runs on it. A field is invisible, it fills space, it pushes without touching, and when fields and electric currents meet in the next two lessons, the modern world of motors and generators falls out.

Lesson 5

Batteries, current, and circuits

Static electricity is a spark and gone. The revolution of the next generation was a flow that keeps going. In 1800 Alessandro Volta, following up Luigi Galvani's twitching frog legs, stacked discs of silver and zinc separated by cloth soaked in brine and produced a steady electric current on demand: the voltaic pile, the first battery. Thompson's Elementary Lessons treats what flows in the new language this course has been building. A current is charge in motion along a conductor, driven by the battery's push, and Franklin's bookkeeping still governs it: the same current that leaves one terminal of the battery returns to the other. Nothing is used up on the way round except the battery's stored energy.

The condition for flow is a circuit, a complete unbroken loop of conducting material from one terminal of the battery back to the other. Break the loop anywhere and the current stops everywhere, instantly, which is the whole secret of the humble switch. Materials divide by how they treat the flow. Metals and carbon conduct freely and make the wires. Glass, rubber, dry wood, and plastic resist so strongly that we use them as insulators, to keep current on its road and out of our fingers. Thompson drills his Victorian students, and us, on the effects that prove a current is present even though it is invisible: it heats a fine wire until it glows, it decomposes certain liquids chemically, and, most important for the next lesson, it makes magnetism.

You already reason about circuits every day without noticing. A torch is a battery, a switch, and a bulb in one loop; when it dies you blame the battery, not the bulb, because the loop's push is spent. A string of old Christmas lights that all die together is one loop with one break. Keep the water picture if it helps, with the battery as a pump and the wire as a pipe, but remember where the picture bends: nothing leaks out of a healthy wire, the flow is the same all the way round, and it begins everywhere in the loop at once when the switch closes. In the next lesson that flowing current meets a coil of wire and an iron bar, and starts pushing machinery around.

Lesson 6

Electromagnets and motors

In 1820 the Danish physicist Hans Christian Oersted noticed, during a lecture, that a current flowing in a wire twitches a nearby compass needle. Electricity and magnetism, the two parlour sciences, were one subject. Thompson states the practical consequence with textbook plainness: a strong current carried in a spiral wire around a bar of iron magnetises the bar powerfully, and “in the case of a soft iron bar, it is only a magnet while the current continues to flow.” Switch on, magnet. Switch off, plain iron. That combination, coil plus iron core plus current, is the electromagnet, and unlike any natural magnet it obeys your switch.

Electromagnets do work everywhere once you look. The scrapyard crane that lifts a car by its roof is one. So is the relay, a small electromagnet that lets a weak current snap a switch for a strong one, the device that ran telephone exchanges and early computers. The telegraph, the first great electrical industry, is an electromagnet clicking at the far end of a wire. And Michael Faraday completed the circle in 1831: if a current makes magnetism, a changing magnetism makes a current. Move a magnet in and out of a coil and a current flows in the wire, with no battery at all. That is electromagnetic induction, and every generator, from a bicycle dynamo to a power station turbine hall, is Faraday's coil and magnet at work.

Put the two halves together and you get the motor. Feed a current into a coil sitting in a magnetic field and the field pushes on the current, the coil twists, and a commutator flips the connection each half turn so the twist never stops. A motor is an electromagnet arguing with a magnet, thousands of times a minute. Run the same machine backwards, spin the coil by steam or water or wind, and it generates the current instead. Franklin began with a spark on a key; within eighty years his science had become telegraphs, dynamos, and motors. When Thompson wrote his lessons in 1881, electric light was just arriving in streets and homes, and his schoolboys were being trained for the wired century you now live in.

Lesson 7

Safe home experiments

This course ends at your kitchen table, with experiments that are genuinely safe because they use only static, magnets, and small batteries. Rule zero comes first, and it is absolute. DO NOT TRY anything involving lightning, storm clouds, kites in bad weather, or wall outlets and mains wiring. Franklin's kite belongs in a history book, not in your garden; Lesson 2 explained why, including the experimenter it killed. Everything below runs on a balloon, a battery you can hold in your palm, and magnets, and even then, treat the rules like a pilot treats a checklist.

Start with Franklin's own century. Inflate a balloon, rub it on a dry sweater, and hold it near a thin trickle of tap water: the stream bends toward the balloon, pulled by its static charge. Roll the balloon slowly above small torn scraps of paper and watch them leap up and cling. Then map a magnet's field the way Thompson's students did: lay a bar magnet under a sheet of stiff paper, sprinkle iron filings or use a small compass moved step by step across the page, and draw the curving lines the filings reveal. Try north pole to north pole and feel the invisible spring of repulsion push your hands apart, then flip one magnet and feel the snap of attraction.

Finish with a battery circuit and an electromagnet. Connect a small torch bulb or an LED with its resistor, if an adult can help you pick one, to a single AA or D cell with two short wires, and find the arrangement that lights it: one loop, no gaps, that is a circuit. Then wrap insulated wire thirty or forty turns around a large iron nail, connect the ends briefly to a D cell, and pick up paper clips with the nail. Disconnect between tries, because the wire warms as the current flows, and never leave it connected. You have just rebuilt, safely, the three pillars of this course: static charge, the field, and the electromagnet. Franklin would recognise every one of them, and he would approve of the checklist.

Sources

The Books Behind This Course

This course is taught from two books, both free for anyone to read in full. Franklin's letters are clean Project Gutenberg text. Thompson's textbook comes from archive.org as scanned page text, so these lessons paraphrase Thompson in the main and quote him only where the scanned line is clean.

• Experiments and Observations on Electricity, by Benjamin Franklin (1751), Project Gutenberg
• Elementary Lessons in Electricity and Magnetism, by Silvanus P. Thompson (1881), archive.org

Graduation

You finished Electricity 101

You charged the jar and balanced its plus against its minus, heard the thunder over Philadelphia and fitted the rod that tamed it, drew a magnetic field in iron filings, closed a circuit, and switched an electromagnet on and off. The sparks in a parlour became the power in your walls, and you know both the wonder and the safety rules. Congratulations: the wired world is yours to read now, one circuit at a time.

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