caution, and safety (proceed). The vanes were illuminated at night by lanterns. Hall disc signals The Hall disc signal (also called a "banjo" signal) was the first electrically-operated signal to be widely adopted by American railroads. Thomas Hall patented his disc signal design in 1867. A banjo-shaped wooden case housed a large iron wire hoop with red silk stretched and glued over it. The opposite end had a much smaller hoop in which a very thin disc of colored glass was secured. This entire iron wire assembly was pivoted inside an electromagnet on what was known as a "Z" armature which was wound with copper magnet wire. When the coil was energized, the wire hoops were moved away from the large glass opening in the front of the wooden "banjo" case exposing its white-painted interior. The colored glass disc at the same time moving away from a clear primitive Fresnel lens at the top of the case which was backed up on the rear side of the case with a kerosene lamp. The disc signal was first placed into service in 1870 on the New York and New Haven Railroad at Stamford, Connecticut, using a track treadle device to activate it, as the track circuit was not developed until 1872 by Dr. William Robinson. The Hall Signal Company installed the disc signals as part of automatic block signal systems, initially utilizing line wire circuits, running on poles alongside the tracks, connecting the track treadle devices. One of the earliest such systems was installed in 1871 on the Eastern Railroad (later the Boston & Maine). About 1500 disc signals were operational by 1896. The all-metal US&S Enclosed Disc Signal was introduced in 1896 and had one version that employed both a red and a green banner (as well as both colored glasses) that were mechanically arranged in such a way as to have the banners and glass roundels exchange places within the signal case as the indication required. Extant mechanical and electrical signals There are examples of various mechanical and electrical signals in several railway museums and in the collections of a very few railroad enthusiasts. These include signals that were manufactured by US&S, GRS, Hall and even the Federal Signal Company. The Hall Company's 1924-introduced variant of the dwarf "Position Color Light" signal (or "PCL" as they were referred to at that time) are among the most rare and sought