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.
“Main line” relays like this unit were common. Typically made with a resistance of 150 ohms, main line relays served on major intercity circuits. According to George C. Maynard, the curator who collected this piece in 1891, "This specimen seems to be a very slightly modified version of the Western Union Relay No. 2 of the Tillotson Company which was introduced in a 150 Ohms model about 1880-1881 at $8.50. As late as 1893 the E. S. Greeley Co. was still using the Tillotson illustration of 1880-1881. However the caption had been changed to 'Improved Western Union Relay'. ... The armature or lever in this model is the old style of two-piece construction. A bar across the poles of the magnet is attached to the lever [instead of a single-piece lever]."
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