The mirror galvanometer was later improved by William Thomson, later to become Lord Kelvin. He would patent the device in 1858.
Thomson reacted to the need for an instrument that could indicate with sensibility all the variations of the current in a long cable. This instrument was far more sensitive than any which preceded it, enabling the detection of the slightest defect in the core of a cable during its manufacture and submersion. Moreover, it proved the best apparatus for receiving messages through a long cable.
The following is adapted from a contemporary account[1] of Thomson's instrument: