When was the transformer invented?

The property of induction was discovered in the 1830's but it wasn't until 1886 that William Stanley, working for Westinghouse built the first reliable commercial transformer. His work was built upon some rudimentary designs by the Ganz Company in Hungary (ZBD Transformer 1878), and Lucien Gaulard and John Dixon Gibbs in England. Nikola Tesla did not invent the transformer as some dubious sources have claimed. The Europeans mentioned above did the first work in the field. George Westinghouse, Albert Schmid, Oliver Shallenberger and Stanley made the transformer cheap to produce, and easy to adjust for final use.