Reed relay

The contacts are of magnetic material and the electromagnet acts directly on them without requiring an armature to move them.

Their current handling capacity is limited but, with appropriate contact materials, they are suitable for "dry" switching applications.

A few million reed relays were used from the 1930s to the 1960s for memory functions in Bell System electromechanical telephone exchanges.

This "Ferreed" method reduced power consumption and allowed both contacts to be used for voice path.

The two windings controlled by the same level were unequal, and were wound around opposite ends of the reed, in opposing polarity.

On each end of the ferreed, the windings provided by the two different P leads were opposed to each other, and the greater one predominated when both were energized.

Despite the sealed contacts, plating with silver rather than with precious metals resulted in reed arrays being less reliable than crossbar switches.

Reed relays have continued in their uses outside the telephone industry, such as for automatic test equipment and electronic instrumentation due to their hermetic seal, fast operate time, extended life to 109 operations and highly consistent contact performance.