A reed switch consists of two ferromagnetic nickel-iron wires and specially shaped contact blades (reeds) positioned in a hermetically sealed glass capsule with a gap between them and in a protective atmosphere. The glass capsule is filled with inert gas to prevent activation of the contacts. The ruthenium or rhodium plated contact surfaces are isolated from the outside environment, which protects the contacts from contamination. Reed switches can be operated using a magnetic field generated by either a permanent magnet or current-carrying coil. They induce north (N) and south(S) poles on the reeds. The magnetic attractive force leads to closure of the reed contacts. Upon removal of the magnetic field, contact open again due to the elasticity of the reeds. There are Reed Switches which operate with permanent magnet, while there are Reed Switches operate with coils which can be magnetized by sending current through them.