Conductors and Insulators

Materials fall into two groups according to whether electricity can pass through them.

Conductors let electricity pass. Almost all conductors are metals: copper, aluminium, steel, gold. The graphite "lead" in a pencil conducts too, and so does salty water — which matters a great deal for safety.

Insulators do not let electricity pass: plastic, rubber, glass, dry wood, cloth, paper and dry air.

You can test any object yourself. Build a circuit with a cell and a bulb, but leave a gap in the loop. Touch the object across the gap:

• Bulb lights → the object completed the path → conductor.
• Bulb stays dark → the object blocked the path → insulator.

Worked example — why an extension cord is built the way it is.

Cut across an extension cord (never do this to a real one) and you would see two materials doing two opposite jobs:

• Inside: copper strands. Copper is an excellent conductor, so the electricity travels along the cord easily.
• Outside: a plastic sleeve. Plastic is an insulator, so the electricity is kept inside the copper and cannot reach your hand.

One cable, two materials, chosen for exactly opposite reasons. Every switch, plug and appliance in your home is built on that same pairing — and it is why a cord with cracked plastic must be thrown out, not taped up.