1928 · Selenium Dry-Plate Rectifier · Work 808 of 1,871
A selenium rectifier stack of 1928 on a pale wooden workbench in a factory in Nuremberg, a compact stack seven centimetres long of about twenty square plates three and a half centimetres across with rounded corners, rust-brown to dark red-brown, with thin dark washers between them, on a central bolt with a bright brass nut and a solder tag at one end, beside it a tin tray of loose plates with a matt grey centre and a bright rim, a charger housing of black sheet steel with its back open, hands in grey smock sleeves setting a plate onto the bolt, skylight from the upper left.
What it was used for
Per the handbook of the Federal Telephone and Radio Company (2nd ed. 1953) the selenium rectifier was first developed in 1928 by a European associate of International Telephone and Telegraph; within a short period 88 per cent of metallic rectifiers used abroad were selenium. Federal introduced it to the American market in 1938 and in 1946 a stack for radio and television receivers. It was used in chargers for 2- and 6-volt batteries, in B+ supplies of home radios, portables, TV sets and record players, for relays and solenoids, in business machines and electric razors, and industrially from electroplating to cyclotrons (about 50,000 volts at 5 mA).
What was new
What was characteristic was the layered construction: no filament, no moving parts. Purified selenium with bromine, chlorine or iodine added is applied under heat and pressure to a roughened, usually nickel-plated aluminium plate and crystallised; over it goes an artificial barrier layer and a front electrode of an alloy such as tin, cadmium and bismuth. The barrier layer itself is formed by applying pulsating DC in the reverse direction at controlled temperature. The barb: soon after 1928 the process held 88 per cent of the metallic rectifiers used abroad, yet needed 10 years to reach the USA - and the radio and television types were rated for a maximum cell temperature of 85 degrees Celsius.
What the sources leave open: The maker's handbook does not name the inventor, only "a European associate" of IT&T; no name is verified here. The figure of more than 35 million units shipped by 1953 applies, per the handbook, to the radio and television series introduced in 1946, not to selenium rectifiers in general.