English: Early portable 9 vacuum tube superheterodyne radio receiver on table, 1923, showing examples of antique radio components used. Built by W. H. Blucock in England and displayed at the 1923 Radio Fair, Hotel Imperial, London, it received broadcast stations from as far away as San Fransisco. The superheterodyne circuit, used in virtually all modern radios, was invented in 1918 by Major Edwin Armstrong while he was stationed with the Signal Corps in France during World War 1. This example was built with early triode vacuum tubes. It consists of a mixer and local oscillator section (left), 5 RC-coupled IF amplifier stages and a detector (right). The low gain achievable with the triode without instability made many amplification stages necessary. The round adjustable coils (left, center) used to tune the set were called "variometers". The long cylindrical segmented component visible in the background is a "B" battery used to provide the plate voltage of 40 - 150 V. It consists of a stack of individual cells, with taps allowing the voltage to be adjusted by cutting in different numbers of cells. The intermediate frequency was probably around 50-75 kHz.