Invented in 1922 by a professor at Leningrad Electrotechnical University [pdf], the reed switch contains two slivers of wire that make contact in the presence of a magnet. Bell Telephone Laboratories evolved this device into the reed relay, in which the switch is controlled by an electromagnet. By the 1960s, reed relays were a fundamental component of telephone exchanges. Their ultralow power consumption also made them popular for space applications, including the Apollo missions.
Although solid-state components eventually eclipsed reed relays for telephone switching, they’re still useful when you need to stop very low currents from leaking away (as in photomultiplier detectors) or are handling high voltages (as in cardiac defibrillators). Along the way, the price of reed switches and relays has come down considerably. A reed switch optimized for aerospace once cost about US $200. Modern reed switches go for well under a dollar, and vintage reed relays like the ones in this ad from IEEE Spectrum’s February 1965 issue can still be found on eBay.
This article originally appeared in print as “Reed Relay Redux.”
Part of a continuing series looking at the story of technology as told through advertisements.