Portable double-needle telegraph and test set, 1850.
© Science Museum / Science & Society Picture Library
This small instrument is both a telegraph and a test set. It was compact enough to be carried by the telegraph lineman when inspecting and repairing telegraph wires. With the ivory stops in position the needles were used to send and receive messages using the standard double-needle code, and when they were withdrawn the needles acted as galvonometers, working over the whole scale.