Closed-core lighting transformers:

In 1878, the engineers of the Ganz Company in Hungary began manufacture of electric lighting apparatus for Austria-Hungary, and by 1883 made over fifty installations. It offered an entire system consisting of both arc and incandescent lamps, generators, and other accessories.

Lucien Gaulard and John Dixon Gibbs first exhibited a device with an open iron core called a "secondary generator" in London in 1882, then sold the idea to the Westinghouse company in the United States.

They also exhibited the invention in Turin, Italy in 1884, where it was adopted for an electric lighting system.
Between 1884 and 1885, Ganz Company engineers Károly Zipernowsky, Ottó Bláthy and Miksa Déri had determined that open-core devices were impracticable, as they were incapable of reliably regulating voltage.

In their joint patent application for the "Z.B.D." transformers, they described the design of two with no poles: the "closed-core" and the "shell-core" transformers. In the closed-core type, the primary and secondary windings were wound around a closed iron ring; in the shell type, the windings were passed through the iron core. In both designs, the magnetic flux linking the primary and secondary windings traveled almost entirely within the iron core, with no intentional path through air.

When employed in electric distribution systems, this revolutionary design concept would finally make it technically and economically feasible to provide electric power for lighting in homes, businesses and public spaces.

Bláthy had suggested the use of closed-cores, Zipernowsky the use of shunt connections, and Déri had performed the experiments.[12] Zipernowsky, Bláthy and Déri also discovered the transformer formula, Vs/Vp = Ns/Np,[citation needed], and electrical and electronic systems the world over continue to rely on the principles of the original Z.B.D. transformers.

The inventors also popularized the word "transformer" to describe a device for altering the EMF of an electric current, although the term had already been in use by 1882.