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A pivoted arm, horizontal for stop and inclined for clear.
Also known as: Semaphore arm, Signal arm, Mechanical signal, Pivoted arm on a post
A semaphore arm horizontal means stop and inclined means proceed, and the design is fail-safe by gravity: if the operating wire breaks the arm falls back to horizontal, which is the restrictive indication. That is the whole reason for the geometry.
Indicates stop when horizontal and proceed when raised or lowered from horizontal, on a mechanically operated railway signal.
Contexts: Public information
Fails safe by gravity: a broken operating wire lets the arm return to horizontal, the most restrictive indication, rather than to a permissive one.
Contexts: Public information
Distinguishes stop arms from distant arms by shape — a square end for a stop signal and a fishtail notch for a distant — which is legible at range where colour is not.
Contexts: Public information
A rectangular arm pivoted at one end on a tall post, held horizontally or inclined, with a coloured spectacle plate behind a lamp at the pivot.