A conductor lets charge move through it, an insulator holds charge in place

1. Metals are the standard conductors: copper, aluminium, steel, silver and gold.
2. Graphite is the exception worth remembering, because it conducts well even though it is a non-metal.
3. Insulators hold charge exactly where it is put, and they include plastic, rubber, glass, perspex, polythene, dry wood and dry air.
4. Tap water and a damp hand conduct well enough to be dangerous, which is why a plug should never be handled with wet fingers.
5. Every mains cable uses both kinds of material: a copper core to carry the current and a plastic sleeve so that the current cannot reach you.

Free electrons are the whole of the difference

1. In a metal, each atom releases one or more of its outer electrons, which are then no longer tied to any particular atom.
2. These are the free electrons, and they drift about at random through the whole lump of metal, leaving behind a fixed lattice of positive ions.
3. Connect that metal into a circuit and the free electrons all drift the same way along it, and that steady drift is the electric current.
4. In an insulator every electron is bound tightly to its own atom, so connecting a supply cannot set up any general drift and no current flows.
5. That one structural difference explains everything else about the two groups.
   1. Charge sprayed onto a polythene rod stays exactly where you put it, because there is nothing free to carry it away.
   2. Charge placed on a copper rod held in your hand vanishes instantly, because the free electrons carry it through the metal and through you to earth.
   3. Metals also conduct heat well for the same reason, since the free electrons carry energy through the metal as they move.

Testing a material is a two-minute circuit

1. Build a series test circuit: a cell, a lamp, a switch and two crocodile clips left as an open gap.
2. Clip the sample being tested across the gap so that it is the only thing that can complete the circuit.
3. If the lamp lights, charge is passing through the sample, so the material is a conductor.
4. If the lamp stays dark, nothing is getting through and the material is an insulator.
5. Swapping the lamp for an ammeter is better, because a poor conductor may pass a current far too small to light a lamp yet still give a clear reading.
6. Use samples of the same length and thickness and the same cell each time, so that the material is the only thing being changed.
7. A charged gold-leaf electroscope gives the same answer with no circuit at all: hold the sample against the cap while gripping its far end, and a conductor lets the leaf fall at once while an insulator leaves it standing.