Inter-continental communications took a giant leap forward in 1858 with the laying of the first transatlantic telegraph cable. It reduced the time taken for messages to cross from the Britain to the USA from days or months to mere hours. But the success of the system relied not only on cable technology but also on an instrument that could detect the tiny electrical signals that passed so far under the sea. William Thomson's device carried a tiny mirror on a coil, which deflected a beam of light on receiving a telegraph signal. It was the only instrument sensitive enough to detect reliably the first transatlantic telegraph messages.

Inv. 1876-68