It was invented in 1922 by professor Valentin Kovalenkov at the Petrograd Electrotechnical University,[1] and later evolved at Bell Telephone Laboratories in 1936 by Walter B. Ellwood into the reed relay.

In its simplest and most common form, it consists of a pair of ferromagnetic flexible metal contacts in a hermetically sealed glass envelope.

The switch may be actuated by an electromagnetic coil, making a reed relay,[2] or by bringing a permanent magnet near it.

When the magnetic field is removed, the contacts in the reed switch return to their original position.

[i] A common example of a reed switch application is to detect the opening of a door or windows, for a security alarm.

The thermal coefficient of expansion of the glass material and metal parts must be similar to prevent breaking the glass-to-metal seal.

The glass used must have a high electrical resistance and must not contain volatile components, such as lead oxide and fluorides, which can contaminate the contacts during the sealing operation.

The reed is hermetically sealed and can therefore operate in almost any environment, such as where flammable gas is present or where corrosion would affect open switch contacts.

Reed relays are used when operating currents are relatively low, and offer high operating speed, good performance with very small currents that are not reliably switched by conventional contacts, high reliability and long life.

At one time brushless DC electric motors used reed switches to sense the rotor's position relative to the field poles.

The motor design could also be "inverted", placing permanent magnets onto the rotor and switching the field through the external, fixed coils.

As cheap Hall effect sensors became available, they replaced the reed switches and gave even longer service lifetimes.

Reed switches may be selected for a particular sensor application when a solid-state device does not meet requirements such as power consumption or electrical interface compatibility.

Wear and life are almost entirely dependent on the electrical load's effect on the contacts along with the properties of the specific reed switch used.