English: Alexanderson alternator an antique type of radio transmitter, placed in the main building of the Varberg radio station in Grimeton near Varberg, Sweden. This was a specialized AC electric generator which turned at an extremely high rate, generating radio waves. They were used between 1906 and the mid 1920s in very low frequency (VLF) radio stations, mostly operated by governments and navies, to broadcast long distances. They were very expensive, inefficient, and finicky to adjust, and were mostly replaced by vacuum tube transmitters by the 1930s.