Bunnell main-line telegraph relay

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. This particular unit is rated for 200 ohms, a somewhat higher than normal resistance. A notation in the record refers to this relay as "R.R. Pattern". Presumably that means the piece was made or used specifically for railroad telegraph lines but the exact meaning is unclear.

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