What is function of closed-core transformer?

Closed-core and shell-core transformer (1885) The development of the electrical industry was hindered by the fact that the dynamo could only supply electricity trouble-free to short distances; to longer distances a large portion of electric energy got "lost" through overheated cables. The electrical engineers of that period were well aware that cheap electrical transmission could only be achieved by increasing the voltage. However, the experiments with DC (direct current) constantly ended in failure. Between 1884 and 1885 three engineers at the Ganz factory, Ottó Titusz Bláthy, Miksa Déri and Károly Zipernowsky developed a new current distribution system based on the use of the induction apparatus called transformer. Their joint patent described a transformer with no poles and comprised two versions of it, the "closed-core transformer" and the "shell-core transformer." In the closed-core transformer the iron core is a closed ring around which the two coils are arranged uniformly. In the shell type transformer, the copper induction cables are passed through the core. The core consists of iron cables or plates. Based on this invention, it became possible to provide economical and cheap lighting for industry and households.