What device would you use to open and close a circuit?

What device would you use to open and close a circuit? A switch. It controls the path available to electric current: when its contacts touch, the circuit is closed and current can flow; when they separate, the circuit is open and current is interrupted.

The familiar wall light switch is usually a mechanical device with movable metal contacts. Flipping it changes the contacts between open and closed states. Switches can also be operated by pressing, sliding, turning, tilting, or sensing conditions such as temperature or pressure. A relay is an electrically controlled switch, while a circuit breaker is a specialized protective switch that opens automatically when current becomes dangerously high.

A common mix-up is reversing the terms “open” and “closed.” An open circuit is broken, so it normally carries no current; a closed circuit provides a continuous conducting path. “On” usually corresponds to closed and “off” to open, although some switches are designed with the opposite normal state. Electronic components such as transistors can also act as switches, but for this general question, “switch” is the single canonical answer.