Which of the following best describes a transformer?

A transformer is fundamentally a device that modifies the voltage of alternating current (AC) in a power system, specifically stepping up or stepping down voltage levels according to the needs of the electrical system. This is achieved through electromagnetic induction, where a primary coil receives AC voltage, creating a magnetic field that induces a voltage in a secondary coil.

The primary function of stepping down voltage is crucial for ensuring that electrical power is transmitted efficiently over long distances and then converted to a usable level for consumer devices. For example, transmission lines carry high-voltage electricity to minimize losses, and substations use transformers to reduce that voltage to a safer, more manageable level for residential or commercial use.

While the other choices refer to functions that are not characteristic of transformers, steppers are specifically designed for voltage manipulation. Generating electrical power is the role of generators, measuring current flow pertains to ammeters, and converting AC to DC is within the domain of rectifiers. Hence, the distinction and function of a transformer as a voltage level modifying device is well established and crucial in electrical systems.