6A In the early days of telegraphy, keys used a brass lever through which a steel pivot shaft was fixed. This made for a heavy key on which the shaft was liable to work loose from the heavy use. In 1881, Bunnell obtained a patent on a "steel lever key" on which the lever and pivot shaft (trunnion) were parts of a single piece of stamped steel. This design became the standard for keys ever since.
The 1881 steel lever ket was originally a leg key; it was later offered in a legless model. Bunnell's best model legless steel lever key was christened the "Triumph." It used, for example, micainsulation as opposed to the fiber of the standard models.
6B The usual telegraph key has a circuit closer and is intended for use on gravity battery circuits. These batteries, typically of the bluestone or crowfoot variety, put out a small, constant current into a closed circit as used on American railroads of the past century.
Dry cells require less mainenance and are not subject to spillage, but would rapidly wear down in a closed circuit. The circuit must be kept open except when keying a message. This kind of circuit (as used in European land-line telegraphy) required an "open circuit key."
6C Another form of open circuit key. Since the dry cell batteries lose capacity quickly under closed circuit conditions, the "circuit closer" on this model actually worked as a cut-out for the battery.
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