Who patented a practical alternating-current transformer in the United States in 1886?

William Stanley Jr. patented a practical alternating-current transformer in the United States in 1886.

Stanley developed an induction-coil transformer while working for George Westinghouse. His design used a closed iron core and separated primary and secondary windings, allowing alternating voltage to be increased or decreased efficiently. He demonstrated the system at Great Barrington, Massachusetts, in 1886.

Transformers operate through electromagnetic induction: alternating current in the primary winding creates a changing magnetic flux, which induces voltage in the secondary winding. The voltage ratio depends mainly on the ratio of turns in the two coils.

Stanley’s transformer helped make alternating-current distribution practical. A utility could raise voltage for transmission, reducing current and resistive losses, then lower voltage for users. Westinghouse later deployed related AC technology on a much larger scale.