For electricity to flow, everything needs to be connected in a big ring. It’s called a circuit. For example, the lights in most houses and flats are part of a circuit controlled by the consumer unit, ...

The simplest complete circuit is a piece of wire from one end of a battery to the other. An electric current can flow in the wire from one end of the battery to the other, but nothing useful happens.

Churning with voltage and resembling an explosion at the wire factory, the breaker panel exudes mystique. But it's just a big switch, filled with other smaller ­switches, which lead to the switches ...