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Note that this article doesn't represent a worldwide view of the semaphore. The semaphore arm consists of two parts: A wooden or metal arm (or "blade") which pivots at different angles, and a spectacle holding coloured lenses which move in front of a lamp in order to provide indications at night.
The red lenses in distant signals were changed to yellow at the same time.
During the 1870s, all the British railway companies standardised on the use of semaphore signals, which were then invariably all of the lower quadrant type.
The advantage of the upper quadrant signal is that should the signal wire break, or the signal arm be weighed down by snow (for instance), gravity will tend to cause the signal to drop to the safe "danger" position.
Some signals converted to electric lamps from oil used a yellow-tinted bulb with the original blue lens to maintain the correct colour.
Materials that were commonly used to make signal posts for semaphore signals included timber, lattice steel, tubular steel and concrete.
To enhance the visibility of the arm, a marking of contrasting colour, such as a stripe or spot, is usually applied.
The rear of the arm is usually coloured white with a black marking.
The Southern Railway in Great Britain frequently made use of old rail for signal posts.
Semaphores come in lower quadrant and upper quadrant forms.
The vertical indication gradually came to be discontinued as the absolute block system superseded time-interval working.
The Great Northern Railway was the first company to introduce "somersault" signals, mounted away from the post, after an accident in January 1876 when a train passed a signal giving a false "clear" aspect because the signal arm had frozen into its slot during a blizzard.
The first railway semaphore signals had arms that could be worked to three positions, in the lower quadrant.