The reed like contacts of a reed relay are encapsulated in a small sealed glass tube that is evacuated or filled with an inert gas like dry nitrogen as illustrated in the figure below:
The contacts are activated by an external magnetic field. The contacts are either dry or mercury-wetted. Mercury-wetted contacts have a thin coating of mercury that fills in surface irregularities, making a larger conduction area and reducing pitting.
In general, reed relays have a long life and low-coil voltage (often, TTL compatible),have faster switching speed compared to electromechanical relays and are immune to dirty environments, nevertheless, they are generally low power (contacts rated at 2 A or less) and vibration sensitive.
Related: Solid-state Relays (SSR) Features and Operation
