By John Papiewski Updated Mar 24, 2022
Developed in the 1930s, magnetic switches function like relays by completing an electrical circuit when exposed to a magnetic field.
Sealed in a glass capsule, they offer lower contact resistance, faster switching speeds, longer life, and eliminate sparking hazards in flammable or explosive environments.
The switch is an elongated glass capsule about 1 cm long and a few millimeters in diameter. Two or more wires pass through its ends. Inside, thin, stiff metal contacts sit a fraction of a millimeter apart and overlap each other. The hermetic glass seal prevents corrosion on the contacts. Simple magnetic switches have two contacts; more complex models contain multiple sets of contacts within the same envelope.
One contact is magnetic; the other is non‑magnetic. A nearby magnetic field from an electromagnet or permanent magnet pulls the contacts together, closing the circuit. When the field is removed, the built‑in spring action opens the contact. Because the contacts are thin and light, the switch can operate up to ten times faster than conventional relays of comparable rating.
Due to the small contact spacing, magnetic switches cannot carry large currents. Currents exceeding a few amperes require a robust metal‑to‑metal connection such as those found in standard relays. Some magnetic switches can handle voltages in excess of 10 000 V, although most operate at much lower levels.
The pull force follows an inverse‑cube law: doubling the distance to a magnet reduces the force to one‑eighth. Consequently, a magnetic switch is highly sensitive to the movement of a nearby magnet. For example, a burglar alarm may mount a small permanent magnet on a door and the magnetic switch on the frame; opening the door immediately actuates the switch.
