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In a lower quadrant signal, the opposite may happen, sending the signal to "off" when in fact it should be illustrating "danger".
Used in conjunction with the time-interval system, the arm horizontal meant "danger", inclined downwards at 45 degrees meant "caution" and the arm vertical (arm hidden within the post) meant "clear".
In the 1920s, the British railway companies began to colour their distant signal arms yellow to better distinguish them from the red stop signals.
The red lenses in distant signals were changed to yellow at the same time.
In a lower quadrant signal, the arm pivots downwards for the less restrictive (known as "off") indication.
Upper quadrant signals, as the name implies, pivot the arm upward for "off".
When combined with the mainly yellow-emitting flame of an oil lamp, this produced a green colour; it was important that the resultant colour was not even yellow-green in appearance, as this could have been confused for a distant signal at caution.
Later signals using electric lamps used green lenses.The vertical indication gradually came to be discontinued as the absolute block system superseded time-interval working.The Great Northern Railway was the first company to introduce "somersault" signals, mounted away from the post, after an accident in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.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.One of the earliest forms of fixed railway signal is the semaphore.