The something you rubbed loose keeps a ledger: two columns, never a third. We did not choose that. We only learned to count it.
⟨ transmission ends ⟩In this fragment you sort every electrified thing into its family, and find there are exactly 2. By the end you will call them + and −, and own the one rule their ledger obeys: charge is never made, only moved.
In The Beach at Miletus, the rubbed amber pulled feathers. Here the amber hangs from a thread, free to swing, and it has already been rubbed on fur. In your hand: a second piece of amber, rubbed on the same fur, the same way. Two stones, prepared identically. Bring one toward the other.
Both ambers were rubbed the same way. As yours comes close, the hanging one will…
Each amber carries the charge one good rubbing leaves, about 20 nC, and the hanging bob weighs 0.5 g. At a 5 cm gap the push tilts it about 16°; the sim computes the tilt from those numbers. The stones are drawn as points for the arithmetic.
The feathers were pulled. This is the opposite: two identically prepared stones push each other apart without touching. Electrified things do not simply attract; alike ones repel.
Glass rubbed on silk is also electrified: it pulls feathers just as rubbed amber does. So the question writes itself. Rub the glass, and bring it toward the hanging amber.
Rubbed glass is electrified too. As it nears the hanging rubbed amber, the amber will…
The glass carries as much charge as the amber, of the other family. Same 20 nC size, same 0.5 g bob, so the pull at 5 cm tilts the same 16°, toward instead of away.
Electrified, yet opposite in effect: the glass pulls what the amber pushes. There is a second family. Charles du Fay found this in 1733 by testing half the objects in Paris, and the sorting he ran is the one you run next.
Four more electrified objects, different materials. How many trays will the sorting need?
Each object carries 8 to 15 nC of one family or the other. Pushed away means the amber’s own family; pulled in means the glass’s. A misplaced object sits wrong in its tray until you move it; the trays keep no secrets.
Two trays were enough, and two is always enough: no object has ever needed a third. The families even obey a lock-step rule: whatever the amber pushes, the glass pulls, without exception. Benjamin Franklin gave the two families their lasting names: the glass’s kind he called positive (+), the amber’s kind negative (−). The choice was an accident of history; swapped, every experiment would read the same.
Why + and −, the names of numbers? Because Franklin saw the families behave like numbers. A thin metal leaf hangs below, neutral. Touch it with the amber and some of the amber’s − moves onto it. Then comes the real test: touch it with a glass rod carrying an equal amount of +.
The leaf carries the amber’s −. You touch it with a glass rod carrying an equal +. Afterwards the leaf…
The amber and glass deliver equal amounts by construction, 10 nC each, so the sum on the leaf is exactly 0. The leaf’s state is read the honest way: by how the amber bob answers it.
Equal + and − on one object add to nothing: the leaf hangs as if untouched. The families sum like +1 and −1, which is exactly why Franklin’s number-names won. And you saw one more thing: before the first touch, the neutral leaf leaned toward the charged rod. Why a neutral thing is pulled at all is its own discovery, waiting in The Conductor and the Insulator.
One question is left, and it is the deepest one. The rubbing put − on the amber. Made from nothing? The fur it was rubbed on is still on the bench. Test the fur.
The rubbing left the amber −. The fur it was rubbed on is now…
The fur carries +20 nC: the same amount the amber took, with the opposite sign. Rub longer and both numbers grow together; the sum of the pair stays 0 throughout.
The rubbing made nothing. It moved charge from the fur to the amber: − collected on the stone and the fur was left one − short, which is what + means. Count both columns of the ledger and the total never changed. That bookkeeping is Franklin’s deepest gift, and no experiment since has caught it failing.
When you are ready, test yourself:
A rubbed rod pushes your hanging amber away. Without another test: what will it do to the charged glass?
You rub a balloon on your hair and the balloon comes away −. Your hair is now…
Two identical leaves carry equal amounts of + and of −. You touch them together, then separate them. Each leaf is now…
You now hold the ledger: two kinds, one unbreakable total. What you cannot yet say is how hard the push and pull are. Next, a fiber twisted by invisible forces puts the first number on them: The Inverse Square.
Lesson 2 done.
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