A Reed Switch or Reed Sensor is a switch that uses glass-enclosed magnetic reeds as the contact members which includes mercury-wetted as well as dry contact types. A Reed Relay is a relay that uses one or more hermetically sealed reed switches. Normally Open (N.O.), or (Form A), contacts represents a state of contacts before any magnetic field is applied to them in which they exist in the open state. Conversely, Normally Closed (N.C.), or (Form B), contacts represent the state of contacts before any magnetic field is applied to them in which they exist in the closed state. A latching relay is a relay that maintains its contacts in the last assumed position without needing to maintain coil energization. To change the state of the contacts, the magnetic field must be reversed.