Relay used at telegraph receiving stations, particularly those operating on long connecting lines. Often signals received over long lines were too weak to operate the receiving instruments directly so the relay served as a very sensitive switch to connect a local battery to the receiving instruments.

A polarized relay incorporated a permanent magnet in its design. Because this type of relay depended completely on magnetic effects, it was faster and more sensitive in operation than a non-polarized relay which incoporated a mechanical return spring.

Physical Description

Black-painted vertical cylindrical metal case enclosing relay mechanism mounted on circular plastic base. Hinged cap at top of cylindrical case. Projecting Mark/Space control knob at base of case.

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