The Conductor and the Insulator

In this fragment you sort all matter into 2 families. By the end you can say which materials let charge move and which hold it still, explain why a charged rod pulls on things that carry no charge at all, and build a box that no outside field can get into.

Every material you have charged so far kept its charge at the spot where you rubbed it. Metals were the odd family: no amount of rubbing seemed to electrify a spoon, and nobody knew why. Stephen Gray found the answer in 1729, and the answer is not that metal refuses charge. On the bench: a metal ball carrying 10 nC, its twin 8 cm away, and 2 bridges in a tray, a copper wire and a silk thread. Lay each one across the gap in turn.

Through copper, the charge moved the moment the bridge touched, and it stopped at the even split. Through silk: nothing, however long you wait. Matter divides into conductors, where charge moves freely, and insulators, where it stays put. This also explains the spoon. A rubbed spoon does charge, but the spoon, your hand and your body are all conductors, so the charge runs through you into the ground the moment it appears. Metal was never dead; it was leaking.

Gray carried his tubes to Granville Wheler's country house, to send the electric virtue, as he called it, down the longest line they could hang: pack-thread, the stout cord used for tying parcels, sagging from post to post to an ivory ball, held up on loops of silk. Send charge down it; then swap a loop for a brass hook no thicker, and send again.

One thin brass hook killed a line that silk had carried for hundreds of feet. So thickness was never what mattered; the material is what matters. The pack-thread is a road for charge. The silk loops are walls that keep the charge on the road. Brass is another road, one that leads down into the earth. Every cable in your house is built on this plan: a metal core to carry charge, inside a plastic skin to keep it there. How different are the 2 families? On the bench: 1 charged globe, 2 pedestals, one copper and one glass, and a clock that runs as fast as it needs to.

The 2 families differ by a factor of about 10²⁰ in how easily charge moves through them. Why metal is full of loose charge while glass holds every electron to its own atom is answered much later on this road, in the fragment The Bands. For now, use the new fact on an old puzzle. In The Two Kinds of Charge, the leaf leaned toward the rod before you ever touched it. On The Beach at Miletus, the feathers jumped before any charge had moved. Both were neutral: equal + and −, adding up to 0. A thing with 0 charge should feel 0 push. On the bench: the detector from The Two Kinds of Charge, a small foil ball hanging on a thread, the kind physicists call a bob, made neutral again with its motes sitting in pairs, and the charged rod.

The bob is a conductor, so its charge is free to move, and the rod moves it. The rod is −, so the bob's + motes slide to the side near the rod, and its − motes slide to the far side. The total is still 0. But the + family now sits closer to the rod than the − family, and The Inverse Square taught you that closer wins: the pull on the near side beats the push on the far side, so the whole bob swings in. This is called electrostatic induction, and it settles the old puzzle: every pull on a neutral thing since the beach happened because the thing rearranged its own charge first. And if you touched the rod to the bob, you saw the ending: some charge jumped across, both were now −, and the bob flew away.

That rearranging is the key to everything else in this fragment, so look at it in its strongest form. 2 charged plates face each other, + on the left, − on the right. The field between them runs straight across at 50 µN/nC, drawn as threads. A metal block waits beside the gap, with a field probe frozen at its centre. Slide the block into the gap.

The moment the block entered, its loose charge moved: − collected on the face toward the + plate, + on the face toward the − plate. And it kept moving until the field of that collected charge, inside the metal, cancelled the field of the plates exactly. Exactly, not roughly, because as long as any field was left inside, it was still pushing loose charge around, which means the moving had not finished. The inside of a conductor goes quiet on its own. It also means the whole block is 1 height on the energy map of The Potential: no field inside, so moving charge inside the metal costs nothing. All of this fits in 1 line:

Read it as a rule, not a recipe: it computes nothing. It says: when the loose charge in a conductor has stopped moving, there is no field left anywhere inside the metal. Mind the words at rest. A wire carrying a current is a conductor whose charge never stops moving, and inside that wire there is a field, pushing. That case opens the next arc of this road.

The strangest consequence is also the most useful one. The inside of your block was quiet, and it held no net charge. So cut the inside out: the field cannot tell the difference, and the quiet stays. The bench has built the hollow box, drawn cut open so you can see in, with a light detector hanging in the middle. Outside waits a 100 nC storm charge, the biggest charge this road has handled.

The walls answered every move you made: their loose charge slid around, − facing the storm and + away from it, always in exactly the pattern that cancels the storm's field inside. A hollow conductor is a room that outside electric fields cannot enter. Michael Faraday built one big enough to sit inside, in 1836, so it is called a Faraday cage. A mesh works too, which is why a car struck by lightning protects the people in it. One secret hides in that perfect quiet: it only works because the push between charges falls with exactly the square of the distance, the law from The Inverse Square. To this day, the sharpest test of that law is to listen for any field inside a closed metal box, and the box has stayed silent every time.

When you are ready, test yourself:

You now hold both halves of a powerful idea: a material where charge moves, and a material where it cannot. Put 2 conductors close together, keep them apart with an insulator, and charge them oppositely: you have built the first machine of electricity, a device that stores charge. Next, The Jar of Leyden.