Telegraph sounders convert electrical pulses into audible sounds and are used to receive Morse code messages. The message travels as a series of electrical pulses through a wire. Short pulses make a dot, slightly longer pulses make a dash. The sequence of dots and dashes represent letters and numbers. The pulses energize the sounder’s electromagnets which move a lever-arm. The arm makes a loud “click” when it strikes a crossbar and the operator translates the pattern of sounds into the original language. The type 1-B sounder was a very common model for main line use. This unit was rated at 400 ohms resistance and was repaired in Western Union's New York Repair Shop, seen by the stamp on the bottom of the unit.
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