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 made by J. H. Bunnell & Co. were one of the most common types of relay and were typically made with a resistance of 150 ohms. As the name suggests, main line relays served on major intercity circuits several hundred miles long. Many relays featured two sets of adjustments, one for moving the coils back and worth, while the other adjusted the spring tension of the armature. The spring was easier to adjust than the coils, sometimes too much so. Manuals from the 19th century emphasized that the springs not be wound around the adjusting screw and several complained of wasted springs due to operators not adjusting the coils instead of the armature. The spring on this unit is stretched and the string is wrapped around the post, indicative of this problem.
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