Reed switches are a type of electrical switch actuated through magnetism. The actuation force can be generated either by introducing a permanent magnet or by a current being passed through an electromagnetic coil. The latter arrangement is key to reed relays.
A simple answer to the question ‘how does a reed switch work?’ is as follows:
Two individual wire contacts, known as the reeds, are housed in a hermetically sealed glass bubble or envelope. These serve as the switch contacts. This bubble is often filled with an inert gas like nitrogen to help prevent particle build-up causing performance issues over time. Depending on the installation environment, the glass envelope may also be encased in plastic for added toughness.
The switch reeds are ferromagnetic, meaning they are made from an iron-based metal that will respond quickly and easily to magnetic forces. This is typically an alloy of nickel and iron - usually 52% nickel. They will often be coated with an additional layer of tougher metal too, for longer working life. For some types of reed switches, only one of the wires is ferromagnetic, and the other will not move. Both reeds are blade-like, designed with a flattened and widened contact area. This offers greater reliability than single-point contacts.
The contacts are sputtered with a microscopically thin layer of iridium, rhodium or ruthenium, a coating which is applied on top of an under-layer made from tungsten, copper or gold. Low-resistivity silver may also be used, and some reed switches utilise mercury. This additional layer is included for optimum electrical contact. As the contacts are wetted, these switches must be held in specific orientations when mounted to prevent the liquid metal from dripping and bridging the contacts when not in use.
When a magnetic field is brought close enough to the outer casing of the reed switch, the contacts inside are pushed together or apart. This makes or breaks the circuit, depending on the direction of movement. In a normally open reed switch - the most common type - the two contact blades are positioned slightly apart, with an air gap between them. This leaves the circuit they are attached to incomplete until a magnetic force is introduced.
In a normally closed switch, the contacts are already touching when at rest. The introduction of a magnetic force instead pushes them apart, breaking the circuit. In either case, the spring force of one or more contact blades returns it to its natural position when not under magnetic pressure.
Lastly, change over switches have three reeds. They operate a break-before-make functionality, meaning that one contact will open its connection to another, before closing connection with the third.