Which device is used to either increase or decrease the voltage of alternating current?

A transformer is a critical device in electrical systems that operates based on electromagnetic induction to alter the voltage of alternating current (AC). It consists of two coils, known as the primary and secondary windings, wrapped around a magnetic core. By altering the number of turns in these coils, the transformer can either step up (increase) or step down (decrease) the voltage while maintaining the same power level, minus losses due to heat and other factors.

This functionality is essential for the transmission and distribution of electricity over long distances, as higher voltages are more efficient for reducing power losses. In practical terms, this means that transformers make it possible to deliver electricity to homes and businesses at a safer, usable voltage.

The other devices noted — a fuse, a recloser, and a circuit breaker — serve different purposes in electrical systems. A fuse protects circuits from overcurrent by breaking the circuit when the current exceeds a certain level. A recloser is a device used to automatically reset a circuit after a temporary fault is cleared, allowing for quick restoration of service. A circuit breaker does similar work to a fuse but can be reset without needing to replace any components. None of these directly modify the voltage level in the manner that a transformer does.