Telegraph relays amplified electrical signals in a telegraph line. Telegraph messages traveled as a series of electrical pulses through a wire from a transmitter to a receiver. Short pulses made a dot, slightly longer pulses a dash. The pulses faded in strength as they traveled through the wire, to the point where the incoming signal was too weak to directly operate a receiving sounder or register. A relay detected a weak signal and used a battery to strengthen the signal so that the receiver would operate.
A plate on this Bunnell relay associates the unit with "Weiny-Phillips" U.S. patent 479178. However, that 19 July 1892 patent was issued solely to Roderick H. Weiny with no mention of Phillips. The relay differs in appearance from the unit depicted on the patent so perhaps Phillips modified Weiny's design.
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