Reed switches (or reed contacts, historically also Herkon) are contact tongues made of an iron-nickel alloy (hermetically) fused into the glass tube, which are actuated by a magnetic field.
The prefix reed (English for tube, reed stalk, North German reed) refers to the glass tube in which the connecting wires are fused.
The former trademark "Herkon" (trademark owner SEL, end of protection 1997, cancelled 1999) stands for hermetically sealed contact.
Reed switches are contained in reed sensors or reed relays. The ferromagnetic switching tongues move in relation to each other when a magnetic field is applied from the outside. This technology makes it possible to produce reliable, hermetically sealed switching elements of small size for - compared to conventional relays and contacts - fast switching operations.
The main components of a reed contact are the switching tongues (paddles) made of a nickel-iron alloy (Ni approx. 48 %) with the outer solder surface (approx. 2-6 µm tin or gold) and inner contact surfaces made of precious metal. A glass tube fixes and protects them and contains the protective gas filling (nitrogen/hydrogen or argon) or a vacuum in the case of switches for higher voltages.
Source: Wikipedia