Here is a little history of the Transformer- A Transformer is a passive electric device that transfers electrical energy between two or more circuits, a varying current in one coil of the transformer produces a varying magnetic flux, which in turn induces a varying electromotive force across a second coil wound around the same core. Electrical energy can be transferred between the two coils, without metallic connection between the two circuits. Faraday's law of induction discovered in 1831 described the induced voltage effective in any coil due to changing magnetic flux encircled by the coil.
In 1885 William Stanley's first Transformer was built. A single phase AC power.
In the picture was Stanley's first Transformer used in the electrification of Great Barrington, MA in 1886.
The Great Barrington System :William Stanley used a Siemens alternator driven by a Westinghouse Steam Engine in a rubber mill. He used a Transformer in parallel to prevent load changes on one device from affecting all other devices downstream. Stanley split from Westinghouse and Established Stanley Electric Company in 1890 nearby Pittsfred, MA. There his company built and sold the first Transformer. In 1903 General Electric bought Stanley Electric Company and continued to build Transformer and capacitors there. The Stanley Electric Company is the root of GE's transformer and Capacitor business. Today there are many companies that build and sell Transformers and Capacitors.
References from Wikipedia and The Edison Tech Center